I have a Ford truck with a 2004 Caterpillar C7 diesel engine at 70k miles (play truck, F750 pickup conversion, mostly in-town driving). I had Rotella 5W-40 synthetic in it with no issues. After upgrading the turbo, injectors, and rerating to a higher power rating; I started to get an oil leak on the ground. I'm almost sure it's coming from the rear main seal. I definitely ruled out the blow by tube. I probably added 8 quarts on the Rotella. For this most recent oil change, I thought I would try a 15w-40 to see if it would slow the leak. I found that Amsoil had a full synthetic 15W-40. It has been about a year now, and I've only had to add about 3 quarts. My question is: Should I credit Amsoil as being a superior oil, or the fact that it's a higher winter viscosity? I'm planning my next oil change, and am highly considering trying a dyno 15W-40 to see what happens.