So how high temps can a board take? How hot does it get in a car in the sun? Will it destroy your board even if the valve is open?

Anyone have any knowledge or experience here?

Most common laminating epoxies soften a little at around 80C but return to normal afterwards. Once they get around 110-120C they soften and do not return to normal afterwards.
So car heat of 60C max is OK.
If anything, a week in the car is not a bad thing for a brand new board as they are not full strength for 7 days and if that was in 20C, well it might not have happened yet.

Never heard of car heat damaging anything when the vent is open but I have seen 3 boards with a fair bit of black paint go pop in the sun just recently as they had no ventilation. So yeah paint it white Dar

Had a board in a bag in a hatchback on a hot summers day. A section in front of the fin box the size is a bread n butter plate melted and sunk in about a cm.
a sloping car window = magnifying glass.

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What Mark says
Plus
I've measured the temp in my car on a hot day it got to 70c , but if the sun shines through the window onto the board it gets hotter, softens the resin and the glass weave shows through. Board bags trap the heat so it gets even hotter in the board bag. I don't use board bags inside the car if I did I would use a white one with the zip undone.

I recall Holden engineers telling me they had recorded temperatures in excess of 80C inside cars.

I think those are areas like close to a dark dash where it meets the windscreen. The vibe of the average over the whole would be cooler.
I always leave windows down a few cm, so even on the crazy hot days it wouldn't be over 60 I reckon.
Do recall a spraycan exploding in some lady's car a few years ago and all the resultant warnings...

I'd say that direct sun on the board is a bigger problem than just general heat. My main board for years was a Starboard Carve 90 which never had the vent screw open. I was carrying it down to the water one day when my attention was drawn to a metre long separation/crack/gaping hole which had opened along one side of the board. Sam Parker, at Wind Surf'n'Snow said it was the 2nd worse case of delamination he had seen. So close, but no cigar.
But back to my point. Heating up the skin using solar power must stress the joint with the polystyrene core of the board. Expansion of one is not transmitted to the other because plastics are thermal insulators, so you get a shearing force at the joint.
Board builders or repairers probably know more than me about that

Is splitting along the seam due to not a lot of overlapping joining top and bottom in modern board making ?

As regards to sunlight on boards I am always amazed at why manufacturers make boards they know will sit on the beach in hot places painted dark colours. Roughly white boards remain cool, all would be best white on the bottom or a very light colour. Most of our boards are white on the bottom( easier when repaired) or light coloured, they sit on the van roof and never have problems. In winter they sit in a container in Gerro( 40deg C quite often outside) with vent plugs in and again we have never had problems, are we just lucky? Touch any coloured board that is is in sun then a white board there is a massive difference!

My surfboard can handle the heat

Na... that would burn my fingers.