Does anybody make a 5w40 non synthetic diesel oil recommended for Cummins. I use Rotella 15w40 for the summer, my truck is still in the break in mileage and I live in very cold climate in Montreal, Canada. The only 5w40 I seem to find is synthetic.
Cummins wants 20,000 miles before synthetic, I have 10k. The owners manual states 5w40 when colder than -18C, therefore I should be running 5w40 in winter however I cannot find a non synthetic 5w40. I stopped by the local Cummins shop and they were also confused but said they always run 15w40 all year round.