It's been in the family since new. There are a lot of things that affect oil change intervals. If you drive long highway distances, you don't need to change as often. Trucks at really dirty work sites should be changed much more frequently. Ask any one who has ever been in the desert for a long period of time.
Synthetic oils have yet to be shown to have extreme benefits, especially related to their cost.
Slick 50 and others have been shown to have SOME affect on engine wear, but the surveys have shown some bias at times. Oh, that NYC taxi thing is another urban myth. The content on this site may not be republished without permission.