You can extend your oil change intervals if you have some sort of aftermarket oil filter and you take oil samples regularly, but you can't just extend your intervals without external filtering. And most recommend still changing the filters at 3-5000 miles, but not draining the oil till the sample says so. But if you go strictly on the sample alone and have gone 19,000 miles on the oil, your asking for a major teardown. I use mobil delvac in both my peterbilt and my sd, I like it alot because it is one of the only oils that does'nt get black immediately after you change it, I can service the pete, and drive 1000-1500 miles before i can't see the dipstick through the oil, and thats working hard pulling heavy loads every day, and idling alot. jmo