Lately I am reading a lot about oils, about their base stock, additive, the whole nine yards. I compared all the numbers and am getting convinced that the universal oil in 15W-40 grade is the way to go, for diesel or gas engine.
Among the popular ones, there are the Shell Rotella, Mobile's Delvac 1300, and Chevron's Delo 400.
Among the three. Rotella is the inferior one (although the most popular). It uses a Group I base stock so it has a higher pour point, lower flash point, and a higher Cold Cranking Viscosity (CCV).
Delvac 1300 and Delo 400 used to be on par. Both used Group II base stock and many numbers are comparable. However, since Mobile switched formulation by using a mixture of Group I and Group II base stocks, Delvac's pour point is higher, flash point is lower, and worst of all, its CCV has increased dramatically. Delo 400's CCV is 3150 cP @ -15 deg C and Delvac's is at 6000 cP @ -20 deg C (it used to be 4000 cP @ -20 deg C). Although the number for Delvac is 5 deg lower but the change from 4000 to 6000 is quite large due to a base stock change.
There are still lots of the old Delvac left in stores but I will not be buying the new formulation.
Chevron Delo 400 is what I will use in my diesel and gas cars, except in the Michigan winter. Delo 400 15W-40 has a pour point of -39 deg F. The Group II base stock really shows how fluid it is at cold. In the winter I will mix 50/50 with Chevron's 5W-40 synthetic, also a universla grade oil, for both gas and diesel engines.