How often should gear oil be changed?

Everyone knows that engine oil in your car needs to be changed regularly but what about the gear oil? Where is it used in a car? Gear oil is thicker oil that is used to lubricate gears. In cars and trucks, it's commonly used in manual transmissions, front and rear axles (differentials) and in some 4WD transfer cases. The most common grades of gear oil are the 75W-90, 80W-90 and 75W-140.

How to find out if your car has any component that uses gear oil? You can check it in your car's owner's manual. Usually at the end of the book, there is an alphabetical index.
Fluid Specifications in Owner's Manual In this Mazda CX-9, the transfer case and the rear differential use gear oil.
Look for "Fluids", "Capacities" or "Specifications." In this section, look for a manual transaxle, transfer case or differential. For example, in this owner's manual for 4WD Mazda CX-9 (in the photo), there are two components that use gear oil: the transfer case and the rear differential (highlighted).

How often does the gear oil needs to be changed? Recommended gear oil change intervals if you use your car for towing or driving in muddy or otherwise severe conditions, vary between 30,000 and 50,000 miles. For normal driving conditions some car manufacturers recommend changing gear oil every 60,000-80,000, while others advise inspecting the gear oil at regular services and change if dirty. Ford goes even further and for some models for normal driving conditions does not recommend changing synthetic gear
oil in rear axles and power take off (PTO) units (transfer cases), saying they are lubricated for life. Of course we know that any mechanical device wears and changing gear oil is a lot cheaper than replacing a transfer case or rear differential. Many suggest changing the gear oil at least every 60,000-70,000 miles or earlier. In general, mineral gear oil needs to be changed more often, while synthetic oil lasts longer.
Rear Differential The rear differential has two plugs, the fill plug and the drain plug.
The gear oil also always needs to be changed if the component (transmission, transfer case or differential) has been submerged in water.

For exact recommendation for your car, check your maintenance schedule. We posted several links to manufacturers website where you can find the maintenance schedule in the bottom of this post. If your mechanic has found a gear oil leak from a differential or a gearbox seal, it needs to be repaired as soon as possible. After the repair is completed, it's good idea to change the gear oil in the affected component. If you are not sure when your gear oil was changed last time, you can ask your mechanic to check the gear oil level and condition.

In some vehicles, there is actually a dipstick to check the oil level in the manual transmission (e.g. Subaru). Without a dipstick, the gear oil level and condition can be checked through the fill hole. Usually, a manual transmission or a rear differential has two plugs: one to fill the gear oil (fill plug) and the other to drain the oil (drain plug), see the photo. In many cars, a mechanic can remove the fill plug and inspect the level and condition of the gear oil through the fill hole. Typically the gear oil must be level with fill hole. If it's lower, it will need to be topped up. If the gear oil looks dirty or has presence of metal particles or shavings, it should be changed. In many vehicles, the drain plug has a magnet attached to it. Whenever the gear oil is changed, the magnet must be cleaned from metal shavings before re-installing.

Changing gear oil in a repair shop is not very expensive: $75-$150 for the front or rear differential and $120-$205 for the manual transmission. It's important to use the proper oil (fluid).

For example, if generic gear oil is used in the Honda CR-V rear differential, it may cause grinding noise in turns.
Honda recommends using specially formulated fluid that can be purchased from a Honda dealer. If a car has a Limited Slip Differential (LSD), it also requires special gear oil. If regular gear oil is used in a limited slip differential, it may produce a chattering noise in slow turns.

Is it possible to reduce grinding in gears by using a special gear oil? Manufacturers don't recommend it, but we know several performance enthusiasts who use special high-performance gear oils in their cars. We have tried using a special synthetic gear oil that acts like a liquid grease (we don't want to advertise the brand) in a manual gearbox with worn-out synchros and it did substantially reduced the grinding while shifting.