DeereFan, I use Mobil 5/40 diesel synthetic in my 2009 GMC Duramax and it does fine. I switched from Rotella dino oil to this. I have not had an opportunity to do an oil analysis but can comment on the following. The engine definitely starts easier (though it has never been difficult) in cold weather. My perception is that it runs a bit more quiet when cold, but that is highly subjective. I have not been able to tell any difference in operating temps or in fuel economy to date--and I drive most days a fairly consistent route. The engine does not use oil either more or less with synthetic v. my prior conventional. I use synthetic oil mainly because I believe it to perform better (again, highly subjective) at temperature extremes and provide 25-50% longer change intervals within the safety window, and I can afford its increased cost. I must admit, however, that I cannot attest that for the average driver of one of these trucks much--if any--difference can be told. Having been a user of both dino and synthetic oil in several diesel trucks so far, I can say anecdotally that I have not been able to tell any difference in fuel economy or performance I can perceive by going with synthetic unless one is in those temp and service extremes noted. Mobil diesel syn is good oil, but one has to personally decide whether it is worth 50% more money for probably 1-2% more performance. Honestly, in my opinion changing a conventional oil at the prescribed intervals probably would do as well for most of us as going with full synthetic and practicing long drain intervals.
John M
I came across Mobil 5w40 turbo diesel synthetic and was wondering if anyone uses it or has had it analyzed. I wouldn't be so concerned if I did not have the 6.0 powerstroke, which from other research thru here and other places is an engine that really needs special attention when it comes to oil.
