I don't know the answer to the original question, but "... this stuff is carried on trucks, so to be safely shippable, it should be able to deal with the heat" made me think.

First, it probably takes more than available daylight hours to heat up a mass of stuff that large. And they usually seem to carry it in white or chrome tanks, which would reflect sunlight.

I wonder if anyone has ever done a temp check to compare the inside of the same model of car, side by side, to see if a white one stays cooler than a black one? Unless the windows would negate any difference...

Sue