I suspect (although it would be fun to do some heat load calcs) the oil cooler dissipates a LOT of heat...lets say 10hp for fun..Now think of a 10HP heater heating 1/3rd of a tank of fuel...I think you can see your fuel would get mighty hot in short order!
Remember, the fuel then has to dissipate its heat to air at say 100F....your fuel could get very warm indeed, especially considering you will evenutally lose a lot of the surface area of the tank cus there will be air between the fuel and the tank.
Gasoline is NOT like kerosene, it has a much higher vapour pressure and will likely flash to vapour at the slightest provocation on a warm day...Really I think a 0.357magnum would be a quicker way to achieve what your likely to end up doing!!!
VL crashes airplanes..it happens its real, don't do it!!
OK..I will now do some calcs when I'm back home from my business trip.
Frank