Tim, you said above "No vehicle manufacturer requires API approved oils", but that post of mine you mentioned..."Only use oils “Certified For Gasoline Engines” by the American Petroleum Institute (API)."
There is even a drawing in the manual of what to look for.
So yes, a car mfg is telling you to use API certed oil.
The manual is very specific, at least for the Stang. For the V6, 5w30 only. Use API certed oil. For the 8, it's 5w20 only.
For my car, it's different. A specific weight isn't listed as it's a non ///M. But it has to be A3/B3 LL-01 under warranty. It can be 0w30, 5w30, 0w40. BMWNA has a very short list of oils for USA driven bimmers.
So it's either Castrol, BMW label, Valvoline MST, or Mobil 1 while under warranty. Nothing else.
Drat, we were typing at the same time.