Another reason the Rotella T4-T6 are marketed as Diesel oils is due to the elevated levels of phosphorus and zinc, which are good for the internals on these old ICEs, but bad for gas-engine catalytic converters. IIRC, Valvoline VR1 and Rotella are in the 1200-1400PPM of zinc and 800-900 PPM of phosphorus which can lead to catalytic converter failure. Don't quote me on the actual numbers, the info is out there if you want to know specifics.