John Wakefiled writes

"The more oil in the crankcase, the better
its ability to remove heat.  In a small engine, the cooling capacity is
increased by 33 pecent and in a large one by about 20 percent by keeping
the oil above the add mark."

Be sure only to keep oil above the add mark and not above the full mark. 
More oil is better only until the crank starts to splash in the pan and 
whisk it into a foam which neither lubricates or cools very well.
I recall an additive from the 60 in the UK from (I think it was) Silkolene 
that reduced foaming and improved cooling. I used it in our beetles to good 
effect. Anyone recall such a product and if any is still available.
Also remember the air cooled engines cooling system uses the viscosity of 
the oil to determine the ratio of flow through the cooler or direct to the 
bearings so using too thick an oil can cause overheating problems. If 
fitting additional cooling capacity in the form of extra oil coolers then 
add some extra oil above the "static" full mark to fill the extra cooler and 
pipes.

Trevor Easton