I use Rotella SAE 40.
Haven't needed different oil in the winter but would use SAE 30 if I needed to.

The couple year old recomendation from Detroit says if below freezing and starting is a problem SAE 30 is ok, and so is 15W-40 but either "must be replaced with SAE 40 as soon as ambient conditions permit"

Again, don’t just go by what I or anyone else says, go by what the engine manufacturer, Detroit Diesel MTU, says to use.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-