I think they contain additives to control oxidation..
I have a Cat diesel with 1,540,000 miles on it...and almost 1,000,000 miles since it was overhauled. I use 15W40 Mobil Delvac oil...no one makes high mileage oils for these HD diesels...makes you wonder why they do for autos...could it be MORE PROFIT?
Nahhhhhhhh...