Downunder,
I can not be 100% sure why they act this way with high oil fuel, it just does.
If I get out my truck and tune it perfectly to run a 20/20 fuel then richen it 1/8 turn on the mixture for my safety margin, it will have a head temp reading about 45 degrees higher than running 20% nitro / 11% oil car fuel that I make, again perfectly tuned to that fuel.
The higher oil fuel will also provide less top end performance and slightly less snap off the line, but I have no problem running it and it still has more power than the truck needs.

I've taken modern aircraft engines such as teh Enya 50 SS and done the same thing, start with 10% fuel with 20% oil then switch to a 10/11 fuel and the exact thing happens, it gets cooler, with all things remaining the same. Obviously it is tuned to the fuel.