I've got 80K miles on my Liberty Diesel. I've used only Mobil 1 0W-40 up til now, but grudgingly, because that's the only brand that carries this wieght. The manual says 5W-40 is the 2nd choice. I found two (1) gallon jugs on Rotella 5W-40 synthetic at Wally World for $11 each . . . . no harm?