There are several Rotellas

Only Rotella T6 is full synthetic. And if he's not running it, he's going to have turbo issues. Unless he's changing it often, running it hard, warming it up and cooling it off, and keeping the PCV and EGR systems clean.

i did not know that! thanks..

I'd still like to know why certain motors require a certain oil wt.

what role does climate play?


KT

Mobil Delvac in my diesel. Cheap at Walmart.

A lot of the new 'thinner' specs are purely for improving mileage. Lots of cars (Ford for sure) call for 5-20. I have heard a lot of people sat 10-30 is better for the same engine.

For climate effects look in your owners manual. It will clearly show the thicker oils for warmer weather. I believe this is almost all 'start-up' viscosity, I don't think running viscosity can change much, as the engine temperature doesn't change.