I'll assume you are comparing Delvac 1 to Mobil 1?
Delvac 1 will do a much better job of suspending and dispersing the soot particles, which will minimize abrasive wear. In addition, Delvac 1 has significantly more TBN reserve, so it will do a better job of neutralizing acids and preventing corrosive wear. Since Delvac 1, 5w-40 flows better than the Mobil 1, 15w-50 all the time, it will improve starting in cold weather as well as fuel efficiency. Delvac 1 will also keep the engine much cleaner, in fact it will clean up deposits left by Mobil 1 in high mileage engines. This isn't opinion, it's based on talking to a number of diesel owners from the 3w.TDIClub.com site ....
I've reviewed oil analysis results from older Mercedes diesels running both Delvac 1 and the Mobil 1, 15w-50, as well as the Mobil 1, 0w-40, which is ACEA B3/B4 rated. Wear rates with Delvac 1 were lower and it held up much better. In fact, iron wear was 50% lower w/ Delvac 1, 5w-40 than with the Mobil 1, 0w-40 in one of the engines I reviewed.
These recommendations would also hold for Amsoil and I'd use their Series 3000, 5w-30 or 15w-40 in ANY diesel engine, even if it's classified as a light duty diesel ....I would NOT use the Amsoil 5w-30 or 10w-30, even though they also carry the ACEA B3/B4 ratings ....
Tooslick
Dixie Synthetics