Vehicle manufacturers spend a lot of money and time on ensuring their vehicles that are properly maintained will last for many thousands of miles, these manufacturers hire top level engineers that understand vehicle fluids and how long these fluids will last under different conditions. One of several fluids that are used in today's trucks and sport utility vehicles is differential gear oil.
Every manufacturer has a recommended service interval for each differential gear oil for normal driving and severe driving. Anyone that tows any type of trailer(boat, travel trailer, landscaping trailer, etc.) or that uses their vehicle for snow plowing will need to follow the severe service interval. These vehicles that tow and plow, or even used off-road, work the vehicle’s differential hard and create a lot of heat which lessens the life of the differential gear oils. When you have the differential fluid serviced, ask your shop if you can look at the fluid as it is draining out of the differential, you will be surprised with a vehicle that may not even be due for this service based on the severe service interval how dark the fluid is.
If you are the original owner and have the differential service performed, look at the gear oil draining and you will see a silver tint as this is the material that gets into the gear oil during vehicle break in period. It is important to know that differentials, unlike transmissions or vehicle engines, do not have filters to remove particulate matter from the gear oil, that is why changing the differential fluids and also the transfer case fluid is essential and good insurance allowing these components to run cooler. A last note is that more vehicle manufacturers are changing over to synthetic fluids, it is highly recommended that spending pennies more per quart is extra insurance to keep these vital components running longer. 