Roy, I used 15W-40 Delo 400 for the same reason virgil uses Rotella. They're both rated for diesel engines and both popular brands. I have no idea how much experience you have or what you know about oils, so I may be preaching to the choir, but use an oil that has an API rating that starts with a "C" (that means it's rated for use in diesel engines); used to be CC/CD, but then later kept moving up the alphabet as improvements came about. I was using CH-4 rating a couple of years ago, but I think there are even later ratings now.