I just bought this 2002 for my son. the seller said that the top end was changed about 7 hour. when i got the bike home it started running rough. i noticed the oil was discolored ( not milky ). i changed the oil and spark plug. bike ran fine but after about 20 minutes of riding the oil was back to being discolored ( grayish brown). any thoughts , suggestions, or fixes would be greatly apreciated.