My opinion is that a 10w-30 is a good all-around viscosity, which Amsoil has in their Dominator & Z-Rod lines. But I always wondered if the Dominator series may have less detergents or a different additive package because it is marketed as more of a "race" oil. 15w-40......maybe, but IMO 20w-50 is unnecessary, unless you need to boost pressure and/or have extra wear or run bearing clearances on the larger side. A good filter and regular oil changes are also important to long engine life with frequency determined by how hard and how often you "abuse" it.

There are other good oils out there, but I always felt that Amsoil offers an extra level of protection, wear and mileage, as seen through personal experience & other sources. Unfortunately, earlier this year I was informed that Amsoil is getting out of the drag racing market, which includes my association with them. Marketing decisions can be difficult to understand, but they are what they are.