Engine: OMV Classic 15W-50, mineral, changed every 500 km
Final transmission: OMV Gearbox 75W-90, synthetic, changed every 1000 km
Brake cables: standard motor oil
Central/side stand and variuos chassis and body parts: high temperature resistance grease and WD 40
I don't know if last two were in question :-)
My oppinion is that full synthetic oil is too much for this engine. I tryed both fully synthetic and semi synthetic(various brands and viscosity grades from 10W-30 to 10W-60), but mineral 15W-50 with often changes proved to be the best in my case.
And about leakage, I can't see how normal fully synth oil can cause this.
Somewhere I read something about diesel oil in GY6 50cc. The thing is that oil for diesel engines has some sort of agressive sulphor cleaning additive (for older diesel engines) that could cause leaking problem on crankshaft gasket/seal on variator side.
Don't know if this is true, but I was using oil for petrol engines and had no leaking problems.