except from the cheap 10W-40 or 15W-40 mineral oil (no synthetic), which is working ok we have good experience with 20W-50 rated oil.
The engine seems to run a little bit smoother and oil consumption is lower as with 10W-40 or 15W-40.
Maybe that is not the best choice if you want to ride during very cold days at minus 10°C in winter, but during spring, summer and autumn it is fine.
Make/brand of the oil is not important.
Clutch and sealings favourite is mineral, no synthetic oil.
In Germany one liter of 20W-50 oil cost from approx. 5 to 6 Euros/liter (and more, of course), if you buy a 4 or 5 liter canister.