I agree. The SM rating is probably not a factor regarding cat conv protection and recommendations. It's that T6 while SM rated, being a 40 grade HD oil is not an ILSAC GF4 oil. I think the ILSAC rating has more to do with gasoline engine cat conv protection than the API service rating, right?
Anyway like you said, his vehicle did not originally require SM or even GF4, but probably GF2 or 3.
Anyway like you said, his vehicle did not originally require SM or even GF4, but probably GF2 or 3.