What's going on here? Rotella T oil (for my Dodge diesel, actually) comes in conventional, fully synthetic and a blend of the two.

Conventional Rot. T - 15W-40
Conv/Synth Blend - 10W-30
Fully Synthetic - 5W-40

It's reasonable that the cold temperature viscosity can tolerate a decrease from 15W to 5W with increasing synthetic since the synthetic is a much better lubricant. Less wear and tear on cold startup.

But why does the operating temperature viscosity for the blend drop to 30 and go back up to 40 for the fully synthetic?

I've noticed this for other brands of oils for diesels. Don't kick me out of here, Cletis! I've asked this on a diesel forum and the post gets hijacked by an oil salesman.


Last edited by Wally / Montana; 02/24/2015 5:43 PM.
1948 3/4-Ton 5-Window Flatbed Chevrolet

34 Years. Now with a '61 261, 848 head, Rochester Monojet carb, SM420 4-speed, 4.10 rear, dual reservoir MC, Bendix up front, 235/85R16 tires, 12-volt w/alternator, electric wipers and a modern radio in the glove box.