Ulf Löfstedt

I just bought a Yamaha Big Bear 350 from 1992. The previous owner has run it on regular engine oil and I have problems with the clutch slipping. I think that's the reason. So what should I do? - Just change oil and it will be OK? - Clean engine/clutch + new oil? - Or do I need a new clutch? It is guessing the centrifugal clutch that slips. It has two clutches , it also has a "regular" clutch . Anyone who has any experience of using wrong type of oil?