I use Castrol 20-50 all the time in the warm months, 10-40 in the winter...works fine but it won't correct valve stem leakage which it sounds like you have. It's really no big deal and the way to tell is this....if the engine is stone cold and the exhaust blows white smoke out then it's coolant and that's not good. If the exhaust puffs alittle dark blue or even greyish smoke out upon cold start that is not so bad but expected. If the engine smokes upon a quick restart when warmed up, white is really bad and blue is not so bad but both represent an issue. As long as the engine is not leaking any fluids visably and burning maybe a quart of oil every 1000 miles I wouldn't worry about. If the coolant is going away that's a problem. Valve stem seals are not a big deal but are typically done as part of a valve job, but as long as you keep the fluid levels up you could drive the damn thing virtually forever....good luck