In my example the MFG specs in writing 5W20, Amsoil says 0W30 is OK. Once again the mfg states in writing 5W20, if the engine fails with oil they don't spec they have the right to refuse the claim. Pretty simple. Then the problems begin. I come from a family with an extensive automotive background. It is not as simple as you think.
Again, I'd rather use RL w/o the Starbusrt than Amsoil w/o the Starburst. Why don't you get that?
Once again, call FMC or Chrysler and ask them if you can use SSO 0W30 in a new car they spec 5W20 for. They will tell you NO.