Chris wrote 

Adam has suggested my high oil temperature could be a timing
problem... can anyone elaborate?

Air-cooled engines and air cooled vans even more so are at risk of damage from heat when the timing is off both to much and to little. timing can over time hours not minutes cook an aircooled engine. the fuel pressure incorrect air fuel ratio, to high of a load, abient air and fan intake temps also play a roll in engine temps. I highly recomend the oil cooler set up from the bus boys to all my customers when they purchase an air cooled type 2/4 engine from me. Oil does not like to acept more heat above 225 F range so when you see higher oil temps the head and piston temps are whats driving the increase. my guess would be that you had head temps in the 450 F range to make the oil that hot and that you where going fast in the heat of the day    

Bob Donalds

http://www.bostonengine.com