What kind of riding do you do? The YZ450s of that era produce a TON of heat, and have small oil capacity/radiators. If you do a lot of trail plonking, they still build up quite a bit of heat. Check your oil often and make sure you allow it kind of "air out" from time to time. They don't like sitting idling.
"The bold adventurer succeeds the best.” - Ovid
WTB: Plated KTM EXC450/525 RFS, Honda CRF450X, Suzuki RMZ450