Oil change time, and I am thinking about standardizing my fleet maintenance. At present only the Kubota diesel tractor uses Rotella T4 15W-40 oil. The Climate Changer (my 1990 F-150 5.8L) is using Rotella T4 10w-30 per the machine shop's recommendation. The 10w/30 is only available in gallons and is much more expensive than the 15w/40, which is sold by the case at Sam's Club for quite a lot less per qt. I am not a chemist but I know the zinc level in the oil is what protects flat tappet cams like the one I put in the F-150. Have also been using the T4 10w/30 in my 2014 John-Deere Gator with no issues, DOHC 3cyl Chery engine. That is the newest engine I am considering changing over. The Gator manual and the F-150 manual both specify API SG or higher. The T4 bottle does not list any S ratings (spark ignition) but is CK-4 rated. If I start using the same Rotella T4 15w-40 oil in all the machines for simplicity and cost savings, will the cams all go flat and the rings gum up? 10w/30 and 15w/40 bottles seem to have the same ratings listed, so surely the zinc is the same in both. I live in Arkansas where it is 100 degrees this week. When it is cold out, I stay inside by the fire so the first viscosity number is of less consequence to me than the hot temp number. Who wants to tell me I'm screwing up?

 

The newest T4 has an SN rating so that should be fine to use in a gasoline application.