http://autos.yahoo.com/maintain/repairqa/tires_wheels/ques125_0.html;_ylt=AqLAjTETYnmrCamQcWlin5eDc78F Found on Yahoo's homepage today: "Do I need to change the oil in the differential? Not usually. The oil inside the differential runs relatively cool and doesn't get very dirty, so it usually is capable of lasting the life of the differential. If your vehicle has over 100,000 miles on the odometer, however, an oil change for preventative maintenance might help your differential see another 100,000 miles." Seems a little ridiculous to me, however I've known many people who have well over 200,000 miles on the original differential fluid. I prefer to change mine out every 50,000-100,000 miles using a full synthetic gear lube.