Just a minor point, but the Rotella T6 is not a diesel specific formula. It is for diesel and gasoline engines and is certified to the API SM spec. It is primarily marketed to the diesel fleets however it has gained significant ground in the motorcycle and forced induction sports cars areas because of its high shear strength and very low wear. Its intended for fleet wide use, not just diesels. I do not think the very low zinc and phosphorous requirements imposed in API SM effect 5W40 and 5W50 weights.I think what he means is the original oil's formula and part number of changed. Due to gov. regulations the level of ZDDP in all motor oils for gasoline engines have be lowered to extend the life of cats. Diesel oils still have the higher levels of ZDDP.