On the early type 1 engines with the oil cooler inside the fan shroud the
cooling air to #3 was bad and VW even tried to play with the timing for #3 to
cool it off. Didn't work so well.

Then they came up with the "dog house" cooler that made #3 get its share of air,
this worked better but #3 still seemed  to get it in the end.

The type IV engine moved the oil cooler around so it was in front of #4 and this
should have worked to even out the cooling but then they loaded up the 3-4 side
of the engine with stuff like EGR, cat & funny heaters. This did not help #3 at
all. The fact that all Fuel Injected type IV engines have asymetrical (un-equal
path lengths from exhaust valves to tail pipe) exhaust systems can not help
either.