ive been useing premium blu e 15w40 conventional oil for the past 15000 miles changing its every 3750 miles. no probs yet.. plus if its cummins rec. and endorsed i cant go wrong. the only thing that bothers me
is the last oil change a few days ago i bought 3 one gallon jugs and the color of all three were diffrent. one was pretty light while the other gallon was really dark???

 

I bought a case of Motorcraft 15W40. Guess that's ok, even though it's Ford approved...