Why would that be? T4, T5, and T6 are all 15W-40 multi-vis oils.Which Rotella? An old NA diesel will be happy with Rotella T4 15w-40.
The T5 and T6, while they are good for newer turbocharged diesels, might
slightly increase smoking and oil consumption in an older, worn engine, IMO.
Why would that be? T4, T5, and T6 are all 15W-40 multi-vis oils.