Michael The answer is YES, absolutely. Synthetic oils are better than conventional oils! They are much more expensive, however. One of the easiest places to read about oils is valvoline.com. I have been using Valvoline 5w-40 synthetic in my D for about 4 years now. I change it every year even with only 2 or 3000 miles on it. It is clean, less drag and an edge against heat damage. I use synthetic gear oil in the final drive and Synthetic in the auto trans. The auto trans is smooth as silk and always looks clean and red. Harold McElraft - 3354