I've been running Amsoil Synthetic Gear Lube. I don't have enough miles to evaluate, but nothing unusual,no problems so far.It comes highly recommended by Aircooled.net.
They sell it if you decide to use and can't find a local source.
Interesting reading:
Go to www.aircooled.net and look at the technical article they have posted concerning Synthetic oil.
Interesting fact:
Harley-Davidson, for the last year or two, recommends their 20/50 fully synthetic motor oil for engine, trans, and primary drive (which houses the wet clutch)for every motorcycle they make.
That might be worth trying.
I've been using it in my 2027 Type 1 engine. So far so good!
Oil temp tends to run cooler with Synthetics too!
Greg B