I have been an Amsoil user/dealer for over a decade, love the stuff..
Castrol is good stuff, but with the reformulation, all oils are going to start to suffer...
I would recommend using "Racing oil" with the needed zinc that is missing from the new formulations...
I also use Comp Cams additive for good measure, I call it cheap insurance.

You cannot do "bad" by changing out old oil. Hell Wesson oil in the crankcase is better than old grungy contaminated crap.

Just remember opinions are like anuses, everybody has one...

I will never push a product down anyone's throat, but hit me up if you want to know more about Amsoil and their products...I don't make a boatload on cash from it, but if I can help you out, lemmeknow....

GT