Detroit Diesel ONLY recommends 40 wt CF-2 oil for their two-stroke diesels. Newell recommends, at least in the '90's model 8V92's, Mobil 1 synthetic oil in 15w-40 grade. If your engine has always run on 40 wt oil, I would stick with 40 wt CF-2. I would NOT run multi-grade dino oil in a DD two-stroke.
It can be hard to find so I would get extra (in diesels, extra means 10+ gallons).
Michael and Georgia Day
1992 Newell 43.5' #281
8V92 DDEC-2, HT740
PT Cruiser GT with Remco Transmission Pump
Thanks....it is hard to come by, and was told by the guy at a local truck stop that 15w-40 would work OK, but I did some of the reading you referred me to, and actually pulled out the Detroit Diesel Operator's handbook, and it specifically says to use 40wt, and to only use 15w-40 in below freezing conditions. I found an online site and ordered 10 gallons of Rotella T 40wt.......