I believe the first change is the most important to get rid of the break in material and the filth typically left in there by the factory. That should come no later than about 5K miles. After that, a high quality oil can last a very long time, as Doug Hillary said. I think his comments were very valid but one must keep in mind the differences in oil capacity between a big truck and a small car. Still, I think after that initial change, in most case a car or truck could go 100K on a good synthetic gear oil in "normal" use. Heavy towing (high oil temps) o r'wheeling (water crossing and dust) would also dictate a more frequent interval. I know that towing at rated capacity in a pickup can bring some high temps in the 275-300 range and long term, that's a killer for any oil (sooner or later). Still, a good oil can easily take 15-30k miles of that, maybe more.
I know the viscosity vs temperature comments he made are valid because I've seen data similar to what he related. Now that I have a very accurate axle temp gauge on my truck and some good baseline temps, his comments have inspired me to consider draining the synthetic 75W140 in my truck's rear axle and installing the same brand of oil but in a 75W90 viscosity and monitoring those temps for a drop. Whaddaya think, Doug?
