[TAKPAK]

I go by what the computer says. Basically I never get the mileage, it's the time. My Duramax says "when the computer says so" or once a year, whichever comes first. As far as what oil, I've never been a proponent of synthetic, with a few exceptions. Some time back, a study was made on synthetic v/s "standard" motor oil. Took some New York taxis and had some with synthetic and some with "regular" oil. Tore the engines down at both 75,000 and 150,000 miles and found no discernable difference in wear. I have used both Chevron Delo 400 15-40 and Mobil Delvac 15-40 with good results and will continue to do so. The dealer does my oil changes, both for convenience to me and for warranty reasons. Right now, they are using Mobil Delvac.