two vehicles, one is my buddies '08 nissan frontier 4.0 V6 with 169k miles and my uncles '05 Ram 2500 Cummins with 324k miles. I service both vehicles and wanted some opinions. My buddy with the Frontier would like to make it last at least 400k. Truck body and engine wise is in excellent shape. I change the oil every 6-7k with 5W30 or 10W30 synthetic or 10W30 diesel oil with a Wix filter and its been running fine, its sees all highway miles. Ive heard that these Nissan engines can be hard on oil, and I was wondering if I should use M1 0W40. We will be doing the first UOA on this truck at the next oil change, I just changed it today with O Reilly HM 5W30 synthetic. My uncles truck has 324k miles, he bought it new, and it looks new, very well taken care of engine and body wise. Wouldnt hesitate to drive this truck across the country. Up until about 225k miles, he had it serviced at the dealer, now that he moved closer I've been servicing it for the last 100k miles. Mobil Delvac 15W40 and a Napa Gold filter every 7k miles. He isnt interested in doing a UOA so i figured this is a safe OCI, he drives mostly highway miles. My question is, while it doesnt get cold down here, should there be any reason to run a 5W40 synthetic in the winter? Would it hurt in a diesel with those miles, or can I stick to the Delvac 15W40. Off topic, He did accidentally fill it up once with gas at a BP station (all green handles) and drove for 60 miles before realizing, engine was checked out and it was fine. That was around 200k miles. Besides that it has been problem free. Time shall tell if my '06 will hold up, it only has 89k, i rarely drive it anymore, just dont want to get rid of it.