I reckon I am pretty good with mechanical stuff having stripped down, rebuilt, overhauled and tuned numerous car engines and drive trains, and my boat engine. I also thought I had a reasonable understanding of oils and viscosity ratings.

Despite this experience I didn't give any thought to the oil I was putting in my boat gearbox. The manual says use the same oil as in the engine so I changed the gearbox oil with the stuff I put in the engine.

My gearbox worked fine when cold but when it got really warmed up sometimes it wouldn't engage the clutch properly. It didn't slip but seemed to grab for a second or two before it engaged fully.

I thought about the oil I was using and had a talk with a diesel mechanic. The engine oil specified in the manual is SAE 30 for Sydney temperatures. The standard diesel oil I pick up from Whitworths or Auto One has a 10-30 rating. The light rating has benefits when the engine is cold and then thickens up to 30 grade when it gets hot.

As the gearbox oil doesn't get anywhere near as hot as engine oil, the oil in the gearbox probably doesn't get near the 30 rating specified for the gearbox.

The mechanic said that the oil could be a cause/contributor to my gearbox problem. He suggested I change the oil before I did anything else.

I changed the oil today and took it for a run. It was only a relatively short run but it got warm and I couldn't replicate the problems despite repeated attempts. It also sounded better. Both of these outcomes might be my imagination but I am pretty confident the oil has made a difference.

Can anybody add to our knowledge of gearbox oils?

Gearbox and diff oils are big deal. Not as much in Australia but Northern USA and Europe.

About 2 decades you can / I noticed / come across new type gearboxes when engine oil is used.
That's great, no need to change for winter season.

I still look after my drive train, but for different reason. my own rule of thumb is
if manual gearbox is design for gearbox oil, after 250k mileage I put thicker oil.
if gearbox is design for engine oil I add oil additives.
/ even to engine after 100k/,
Back to your question, not an expert but this is a wild guess.
An engine oil is very thin compare to gearbox oil. Considering wear and with longer run,
means it gets warmer and thinner.
Fresh oil has always higher viscosity, no problem in short run.
You need a bit more viscosity, either way add oil additive or a touch thicker oil.

It is going to depend whether you have a hydraulic gearbox or a mechanical one. Hydraulic boxes will have either hydraulic fluid or thin single grade engine oil. Mechanical boxes usually have single grade engine oil. I would certainly not use a multi grade oil. I have a multigrade oil in my engine simply because its easier to buy but I would prefer a single grade 30. Same for my Volvo box. The ratings of the oil in the box and engine is important. Don't go above what's recommended, you may have to shop at a tractor place!

My gearbox only has 40 hours on it. It has a plate attached saying: "USE ATF OIL ONLY". (automatic transmission fluid)

I checked the manual which specifies SAE 20 OR 30 oil.

I'm guessing the plate with ATF was fitted by the people who fitted the engine and gearbox, (ten years ago) but I can't understand why.

I think I'll follow the manual which specifies an oil change after the first 50 hours and drain the oil and replace with SAE gear oil.

Engine oils contain detergents which are designed to capture the products of combustion, that's why it turns black. I wouldn't use it in a gearbox.

EDIT. I've just checked what's in there and it does look like ATF.
The gearbox is a TWIN DISK MARINE TRANSMISSION MODEL MG 340

In a gearbox use a single weight gear oil, which oil depends to a great extent on the operating temperatures and ambient conditions. A gearbox oil would normally be above a 50W weight and contain ingredients to handle extreme pressure and be designed purely for gearbox use.

In a marine diesel engine, unless it is of multi valve and multi cam shaft configuration the same applies, a single weight oil would suffice.

Multi weight oils don't change their viscosity, but rather it's how they behave eg. 10W-20 and a 10W-40 have the same viscosity but one behaves like a 20 weight while the other like a 40 weight. My car with 32 valves and four cam shafts specifies 10W-30, my motorbike uses 5W-30, the bike is a lot harder on it's engine so requires more protection and a 5 weight oil will circulate faster on start-up.

As Shotchas has said engine oils contain detergents ..... use those oils in your engine as they prevent sludge formations and keep the internals squeaky clean. But like most things, that's only true if you actually do the oil change at the specified times.

All good points and with similar views. I have an 1954 Triumph sports car which also specifies engine oil but on closer inspection it asks for a 30 grade. On the recommendation of the owners in the club with much more expertise in these things I use Castrol Edge 25-50, apparently it has the right ingredients for the overdrive clutches and is the right weight.
It is a stupid mistake to put the wrong oil in the gearbox. I think I was fortunate that I had a seized gear cable which I changed and then thought through the operation of the gearbox to arrive at the conclusion it had the wrong oil. I had done no more than 15 hours before I changed the oil last weekend so hopefully no damage done.

If it was mine I would use what ever is listed in the owners manual which you say is SAE30 for your temperature range

Regards Don

Check out this link, this guy reckons the gear box should have auto transmission oil.



Bill

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Propwalk said..
Check out this link, this guy reckons the gear box should have auto transmission oil.



Bill

I would be reading your engine / transmission manual to see what the manufacture recommends

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Charriot said..
Gearbox and diff oils are big deal. Not as much in Australia but Northern USA and Europe.

About 2 decades you can / I noticed / come across new type gearboxes when engine oil is used.
That's great, no need to change for winter season.

I still look after my drive train, but for different reason. my own rule of thumb is
if manual gearbox is design for gearbox oil, after 250k mileage I put thicker oil.
if gearbox is design for engine oil I add oil additives.
/ even to engine after 100k/,
Back to your question, not an expert but this is a wild guess.
An engine oil is very thin compare to gearbox oil. Considering wear and with longer run,
means it gets warmer and thinner.
Fresh oil has always higher viscosity, no problem in short run.
You need a bit more viscosity, either way add oil additive or a touch thicker oil.

Nice post, Charriot, I'm learn something every day.
Can you give an example of an additive product you have used to increase viscocity (for example, to add to SAE 30 oil)?

The trouble with the manual Donk is that it isn't that clear. In the text it says to use engine oil. In the specs it gives a range of mono grade engine oils from 10 to 30 depending on average local temperatures. Above 20C it says use 30 grade, below 20 20 grade and below 10 10 grade.

According to the specialists 10/30 in the engine is OK as it is in the range specified, flows better at start up than a 30 grade but thickens up when hot.

My understanding is that the gearbox doesn't get that hot so the oil doesn't thicken up the same. It needs mono 30 grade.

Also bear in mind that some gearboxes, probably only in older cars and maybe also in boats, use gear oil at 80/90 grade.

The comments above offer good advice. The message is make sure you know what the gearbox really should have.

Karsten, I think the products you are asking about may be some of these ......

www.lucasoil.com.au/products/engine-oil-additives

Or maybe this one .... I use this one in my engines

www.itwpf.com.au/molybond_lubricants/product.aspx?productid=69

There are of course others as well .......

This should settle it .....
The Society of Automotive Engineers (SAE) established a numerical code system for grading motor oils according to their kinematic viscosity.

The viscosity of single-grade oil derived from petroleum unimproved with additives changes considerably with temperature. As the temperature increases, the viscosity of the oil decreases in a relatively predictable manner. On single-grade oils, viscosity testing can be done at cold, winter (W) temperature to grade an oil as SAE number 0W, 5W, 10W, 15W, 20W, or 25W.

The temperature range the oil is exposed to in most vehicles can be wide, ranging from cold ambient temperatures in the winter before the vehicle is started up to hot operating temperatures when the vehicle is fully warmed up in hot summer weather. A specific oil will have high viscosity when cold and a low viscosity at the engine's operating temperature. The difference in viscosities for any single-grade oil is too large between the extremes of temperature. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers, or VIs are added to the oil. These additives make the oil a multi-grade motor oil.

10W-30 designates a common multi-grade oil. Historically, the first number associated with the W (again 'W' is for Winter, not Weight) is not rated at any single temperature. The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped. The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature.

Morningbird, the oil does not get thicker as it heats up, it gets thinner. Please consider this when choosing an oil to put in either your engine or gearbox.

L Ch precise explanation, back real life.
I personally would not call it gearbox, perhaps it is close to transfer case.

it is quite obvious that any wear and problems related, can be eliminated
increasing viscosity of the oil.
As a prevention and limited cure of the problems, before radical overhaul.

In car gearbox, type of oil is more critical, in my old Jackaroo gearbox oil
is the engine oil, otherwise, can't change gears.
Transfer case, at least any oil will work.

additives, I would not do any oil change without it
use any oil additives, even cheap smoke stopper,
has enough molybdenum to guarantee to work
now, for diffs and transfer case I use LSD oil only.

Wow, thanks LooseChange, wish I had the time to do all that research!

SUV manual gear boxes lack oil circulation worst when towing the needle rolls don't get enough oil circulation and over heat that was my thoughts 10 years ago may have changed do not know now been out of the part of the industry for a while

The Concise Oxford Dictionary, 1956:-

"viscosity, noun. Stickiness: (Physics, of fluids, semifluids & gases) internal friction, power of resisting a change in the arrangement of the molecules."


In high school physics and at the Navy Engineering Apprentice School I was taught that "viscosity" is a measure of fluidity in that Ice is not viscous and Water is very viscous or that honey is less viscous than water.

Therefore one would assume that a "high" viscosity oil is thinner or more runny than "low" viscosity oil. However whichever internet site on the topic I find contradicts that.

Regardless of all that, what is important when selecting an oil is it's "film strength" in the machine's operating conditions.

With marine gear boxes, usually if it has multi plate clutches, Auto Trans Fluid will be the correct lubricant. If it has cone clutches, engine type oil would be correct to use.

The gear box manufacturer's specs are best to follow and not necessarily the engine manufacturer's specs.

One thing I do know is that water will be very vicious in a gear box.

A few sites here referenced to oils:-

www.racq.com.au/cars-and-driving/cars/owning-and-maintaining-a-car/facts-about-fuels/understanding-engine-oil-designations

www.upmpg.com/tech_articles/motoroil_viscosity/

www.driverstechnology.co.uk/oils.htm

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nswsailor said..
Wow, thanks LooseChange, wish I had the time to do all that research!

Google can be your friend .... if you let it

You are not imagining the clutch slipping! I had the exact same experience. I have a Bukh DV10-LSME and when I slipped the boat for the first time I read the oil spec for a different Bukh gearbox that required oil of the same rating as the motor (SAE 30 mono grade). After a lot of motoring over Christmas up on the harbour I started to hear what was a slipping noise when accelerating from zero to full revs. Like a screeching sound.
I was convinced the sail drive was on its way out, and even called our shipwright to book in a slip.

Later when I discovered the correct gear oil 85-140 and replaced it at the next slipping the noise went away !

You are not imagining the clutch slipping! I had the exact same experience. I have a Bukh DV10-LSME and when I slipped the boat for the first time I read the oil spec for a different Bukh gearbox that required oil of the same rating as the motor (SAE 30 mono grade). After a lot of motoring over Christmas up on the harbour I started to hear what was a slipping noise when accelerating from zero to full revs. Like a screeching sound.
I was convinced the sail drive was on its way out, and even called our shipwright to book in a slip.

Later when I discovered the correct gear oil 85-140 and replaced it at the next slipping the noise went away !

There seems to be a lot of confusion here where there is no need. First of the gearbox in a boat has virtually nothing in common with car gearboxes. Once a boat box is in gear that's pretty much it.

Some of us have mechanical boxes and others hydraulic. They are not the same. Some have saildrives but they are just mechanical boxes with extra bits. It does make a difference if you have cones and not the right oil. Adding stuff to make the fluid more slippery is not going to help if the cones get polished.

Use what the manufacturer says. If your manual says mono grade, do not use multi grade. Some hydraulic boxes will use a thin mono engine oil or ATF or hydraulic fluid. Ones vegetable based and ones mineral.

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Ramona said..


Use what the manufacturer says. If your manual says mono grade, do not use multi grade.

So, in the interests of improving the forum's collective understanding of engine and gearbox oils, what exactly will happen if you use multi-grade when mono-grade has been recommended?

Technical answer anyone? Technical insight highly preferred, rather than diffuse motherhood statements like "engine will wear quicker" or "will corrode".

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Karsten said...

Ramona said..


Use what the manufacturer says. If your manual says mono grade, do not use multi grade.

So, in the interests of improving the forum's collective understanding of engine and gearbox oils, what exactly will happen if you use multi-grade when mono-grade has been recommended?

Technical answer anyone? Technical insight highly preferred, rather than diffuse motherhood statements like "engine will wear quicker" or "will corrode".

Use what is recomended in your manual for both engine and gear box

Here's a couple of quotes from the internet to give a bit more technical understanding of the issues:

Quote 1: Multi-grade oil: "As the oil wears in use, the viscosity modifiers also wear (or shear to be specific) and the oil gradually loses some of it high temp capability. For example a 10W40 will degrade over time to perform like a 10W30 and then a 10W20 until it is effectively a 10 weight with no high temp protection."

Quote 2: Mono-grade in Yanmar marine diesels:
"When Yanmar wrote that manual about 30 years ago, multigrade oils were somewhat new and didn't have as much lubricity as single grade oils. That is still the case (except for synthetics) but the gap has narrowed considerably. You can use either straight 30 wt or 15W-40 multigrade. Newer Yanmar engines specify use of either. "

Looking at the above, it would seem that if you replace the oil per regular schedule, before the viscosity modifiers "shear", a multi-grade could be viable because it protects the cold engine on start-up, and then the hot engine later on, whereas the mono-grade oil has only a single temperature protection range.

But there may be OTHER TECHNICAL factors at play.

So more technical insights welcome ...

No body is discussing engines here. The question was about gearboxes. The fluid is either transmission fluid or mono grade engine oil. Some boxes can run either, but follow the manufacturers recommendations.

Not sure that Loose Change post above isn't contradicting itself.

"The "10W" means that this oil can be pumped by your engine as well as a single-grade SAE 10 oil can be pumped. The second number, 30, means that the viscosity of this multi-grade oil at 100°C (212°F) operating temperature corresponds to the viscosity of a single-grade 30 oil at same temperature. Morningbird, the oil does not get thicker as it heats up, it gets thinner. Please consider this when choosing an oil to put in either your engine or gearbox."

The 10 grade cold oil is what I termed "thinner" and therefore flows better when cold. As the 10-30 multi grade oil gets to operating temperature its viscosity is that of a 30 grade oil. Multi grade oils with their additives increase in viscosity to better protect the hot rubbing surfaces.

The point of my post is the viscosity (loosely related to the thin/thick nature of the oil) increases with temperature in multi grade oils. Therefore, if you use a multi grade oil in the gearbox, as I did, and it doesn't get to operating temperature then the viscosity will not achieve the higher rating. i.e. a 10 30 won't get to a 30 grade viscosity unless it gets to operating temperature which won't happen in a gearbox.

My gearbox problem wasn't slipping, it was the clutch grabbing for a few seconds as it engaged causing the engine to rock on the mounts. It doesn't slip even at full power once the gear is engaged. This what lead me to investigate the oil.

He is a explanation of how oils are rated www.upmpg.com/tech_articles/motoroil_viscosity/

Regards Don

How could one argue with this?

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Karsten said..

Ramona said..


Use what the manufacturer says. If your manual says mono grade, do not use multi grade.

So, in the interests of improving the forum's collective understanding of engine and gearbox oils, what exactly will happen if you use multi-grade when mono-grade has been recommended?

Technical answer anyone? Technical insight highly preferred, rather than diffuse motherhood statements like "engine will wear quicker" or "will corrode".

I have repaired engines that have been run using the wrong oil grade. Engines that run hydraulic lifters and other hydraulic systems with in the engine can run into trouble unless the correct grade of oil is used.
Eg: I have had to remove the hydraulic lifters from engines, compressed them to remove the oil and refit them. The wrong grade oil in them was causing the lifters to hydraulic up solid, the valve springs where unable to over come their pressure and left the valves to ride open, losing compression in the cylinder and ultimately stopping the engine. The riding of the valves also caused excessive carbon build up in the ports and could have gone on to damage valves and seats.
The symptoms of this was a motor that started perfectly, ran for a while then stopped, cooled down then started perfectly again.

One of the Japanese v 6 motors is a classic example if what u said above wrong oil won't start or very difficult to start

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MorningBird said..

The 10 grade cold oil is what I termed "thinner" and therefore flows better when cold. As the 10-30 multi grade oil gets to operating temperature its viscosity is that of a 30 grade oil. Multi grade oils with their additives increase in viscosity to better protect the hot rubbing surfaces.

The point of my post is the viscosity (loosely related to the thin/thick nature of the oil) increases with temperature in multi grade oils. Therefore, if you use a multi grade oil in the gearbox, as I did, and it doesn't get to operating temperature then the viscosity will not achieve the higher rating. i.e. a 10 30 won't get to a 30 grade viscosity unless it gets to operating temperature which won't happen in a gearbox.

Have to agree with that. Also gives the best info I have seen so far as to why a multi-grade could harm a typical marine transmission - if the transmission runs at sub-80C temperatures the multi-grade acts as a mono-grade but with the wrong viscosity. Well put Morningbird.