It should say on the cap what viscosity it should run. Mine calls for 10-30. If that's the case with yours and the 10-30 is rated for use with diesels, it should be fine, but there are better choices. I run synthetic rotella 5-40 in mine. I'd be Ok running 15-40 non synthetic as well. You need to make sure it's rated for diesels though. Diesels need more detergents and other additives.
