New member, first post. I drive a Dodge/Cummins and live close to a Chevron jobber who keeps a couple cases of Delo 400 5W40 in stock. As a member I buy it for $13.00/gal. It is a CI-4 rated oil and can be bought in quarts - drums. There is also a 0W40. I run the 5W40 year round in So Cal where I tow in 100+ degree heat. After reading some old posts I began to wonder if this is the oil I should be using. Should I be using a dino 15W40 or go to something like DELVAC 1 that I can buy for $18.00/gal.?