I would disagree about synthetic oil causing higher head temps on air-cooled
engines.
Why?
I have a custom built T4 2L engine.  The head temps ran @250 to 275 degrees
with synthetic (275 during winter fuel).  I have switched to Bosch quad tip
plugs from the single and the temps went down to 200 to 240 now still with
synthetic oil.

I ran for a short time the copper three prong plug and the head temps soared
to 300 to 325 degrees.  I checked the timing and such, but still high so I
went to single tip and temps dropped back down to 250 to 275.  I switched to
quad tip and temps dropped to 210 to 240 degrees.

The quality of the engine and the rebuild and other factors determine head
temps more than oil type.  IMHOARWE

Robert

Robert