You don't think the dealer would balk at 15w40 if push came to shove??MAKE SURE you double check your oil level after the dealer replaces your oil, my local dealer over filled my truck every time I got my oil changed. It got so bad that I had one free oil change left on my contract when I decided to changing my own oil. I use Amsoil 15-40 diesel oil. I run stock oil filters because I do want to give the dealer ANY reason to deny my warranty if I have a problem.