I used 15W40 in my Cummins 14 Liter as well as my 10.7 3engines for over 35 years. I won't mention the brand. I sell Rotella as well as other brands. I had been changing oil at 15,000. I recently switched to synthetic blend and my oil samples still look good at 20,000, And the gauge shows pressure noticeably quicker on cold starts. I cm contemplating switching to full synthetic in my trucks. Possibly buying a 55 gallon drum instead of of 300 gallons in bulk. I use 5w30 full synthetic in my GMC 6 liter.
European engines are very strict about speccing 0W40 and thats what I use in my European engines. Europeans put a lot more effort into engineering instead of meeting emissions like we do.
I will not take a step back to petroleum based lubricants and will probably step up to full synthetic. I have been using synthetic in my transmissions and rears for at least 25 years. We used to rebuild them every 200,000 miles now they go over a million miles if nothing stupid happens to them.
