I've read and heard that oil never wears out, it just becomes contaminated (carbon which changes its color, metal particle contaminates, fuel, dirt, etc.) so it can't properly lubricate. I'm still not sure If I believe the "never wears out" statement but it easly becomes contaminated which removes it's ability for proper lubrication. I use Royal Purple in everything on my 4Runner. The engine oil gets drained every 10k and I use a K&N 3001 oil filter which is also changed every 10k. Just use a good filter along with a good synthetic and you'll be fine. It helps to use a good air filter too to trap as much incoming dirt as possible.