The 15w40 is a little heavier, and considered heavier duty. It is use a lot in diesel OTR tractors and diesel autos. It is a little thicker when cold, so it might not flow as good until it warms up. I like to stay with what the owner's manual says for my stuff but 15 40 is used quite a bit in hard working diesel motors so the dealer is probably comfortable with recommending it. If my manual gave me a choice of 10w30 or 15w40 for my diesel, I would go with the 15w40, especially in the hot summer months.