If you look up viscosity weights you'll see that some 0w30, 5w30, and 0w40 are all about the same viscosity at operating temp.
I think Mobil 1 0w40 might even be thinner than Castrol 0w30, but they are mostly close to ideal vis.
Shell Rotella T6 is a diesel oil with good additives for all that carbon, so will work great with gasoline engines, too.
It seems that there are many oils out now that won't hurt engine, but aren't necessarily spec'd for them.
|