This is opening up a can of worms but I honestly think the bike is not that temperamental, it is based on 1980's technology afterall.
I've run numerous oils including leftover "top-up" oils from the cars (Mobile 1, Valvoline, Pennzoil, WalMart brand) and really, they all felt great in the beginning and started to make the shifting "sticky" as their lives wore on. I will admit though that the higher grades: 15-50 and the like did not burn off as fast.
This is in the US mind you: If I were to go out and buy it (if I were being picky), I'd either get Rotella T synthetic or I know the Pep Boys has Castrol 4T on the shelf. Unless otherwise, whatever is on sale and just change it sooner than 5k. Fresh oil is better than fancy-pants old oil.
Unless you're the original owner of the bike and it's the original clutch, I'd say deglaze the discs and run it. I know my bike was the first for several owners so the original clutch as a bit tired.