I've been wanting to do some updated researching of engine oils for use in the DMC. Lots of info & mis-info/wives tales & the like on the internet. It makes it hard to be confident of anything. Bottom line, I've been using Mobil 1 15W50 with great results but I'm thinking of trying Shell Rotella T 15W40 or Chevron Delo 15W40 (both are rated for diesel use & are heavy duty oils). Since I took the time to research the information, I thought it would be good to share it here for others to benefit from a single source of info.

Basics: the DMC manual calls for 10W-30, 10W-40, 15W-40, 20W-40, for my weather conditions (southern PA) & 20W50 for weather over 64F. Of those, I understand the 10Ws are not recommended because they are a bit light at startup. I tried 10W30 Mobil 1 one season & the engine was noisier until it warmed up.

DMCH recommends Castrol GTX (conventional) 20W-50. I believe Dave Swingle at DMC Midwest likes Mobil 1 15W-50 (full synthetic) which is what I have pretty much always used since I got mine in 2008. My engine has been spotless when I've been inside it, so I know it keeps things clean.

The big argument is that we have a flat-tappet design for our valve lifters & it's metal on metal wear. Many think that we need ZDDP (Google it) levels of 1000ppm or higher. These 3 links provide decent detail in a brief read.


Then there are these 2 posts of overlapping data of a guy doing independent testing showing the actual zinc & phosphorous levels as well as a wear pressure rating. Of the oils discussed, they rank in this order from highest wear pressure protection to lowest. All are good, but you can see the differences. I pulled out the commonly available oils from the list that are also in the DMC owners manual (excluding the 10W oils). Notice that the 4 conventional oil have better wear pressure than the synthetic Mobil 1. DMCH's recommendation appears to have the best pressure wear rating & the 3 popular diesel oils are very similar. I find it surprising that the Mobil 1 full synthetic is the lowest of the group, but I still consider it a great choice.
  1. 20W50 Castrol GTX conventional, API SN = 96,514 psi
  2. 15W40 CHEVRON DELO 400LE, conventional, “Load Carrying Capacity/Film Strength” = 73,520 psi
  3. 15W40 MOBIL DELVAC 1300 SUPER conventional, API CJ-4, CI-4 Plus, CI-4, CH-4/SM, SL = 73,300 psi
  4. 15W40 SHELL ROTELLA T conventional, API CJ-4, CI-4 Plus, CH-4, CF-4,CF/SM = 72,022 psi
  5. 15W50 Mobil 1, API SN = 70,235 psi



Here is some more if you want to dig deeper at some of the better information I found when searching.