Hi Mallet, I had a similar problem with a Suzuki that I bought, not been used for some years and the plates were welded together, I had to dismantle the clutch and hammer the plates apart, they would not break apart even with the engine running and the rear wheel spinning, sometimes you have to just take them off and fix the problem, not sure I understand about the car oil but if someone has put car oil in the engine then the plates are buggered I'm afraid as car oil has additives that will wreck the cork plates on your bike, loads of folks try car oil thinking it's a cheaper oil but don't realise that it's meant for car's not bikes. Someone might be able to tell you how to clean them off but I think that once they are contaminated you have to replace them,l sure that's not what you want to hear.