I am purchasing a S500 1996. The engine metal parts, mainly those easily seen beside the big black cover, has a yellowish discoloratiion. I have seen this on some other 500 and 420 engines ofthe same vintage. But others have no discoloration. Is this condition a problem or indication of a problem of any kind. If it is I may want to cancel my purchase. Any reply by tomorrow 7/18 would be greatly appreciated. Thanks.