As iv said before my brother and I have 7.3 powerstrokes in our Ford trucks, my brother runs Rotella synthetic 15-40 in his, he says it starts easier in the winter as he plows snow for people, I have used regular 15-40 I would get in 55 gallon drums made by cam 2, our use tsc brand or Rotella T, so tsc had a rebate for Rotella $5 back per gallon, well after reading the rebate it's only for the synthetic oil, well I figured I'd use that in my truck this time, I needed 5 gallons, and they only had 3, so I went back and forth between Rotella t and traveller brand, I finally just got the traveller oil, never had a problem with it in the past! I still have to go to another tsc tomorrow as this one was out of hay stretcher. Lol, they may have the synthetic oil, now I remember years ago they said not to mix synthetic with conventional oil, then a few years later they came out with a blend? Same with 2 cycle they said use one or the other, my brother uses Amsoil I use shell nautilus or stihl, but iv used his Amsoil in my saws, and went back to my mix, no problems, what do you guys prefer to use,synthetic or conventional?