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How Long Does Gear Oil Last?

Understanding The Basics Of The Most Neglected Lubricant In Our Diesels

Text By , Photography by Diesel Power Staff

Sometimes months or even years can go by before diesel truck owners think of their axles’ gear lube. Although the intervals between axle oil changes can be stretched as long as 50,000 miles in some cases, under severe use (such as towing heavy), it’s a good idea to change your gear oil every 15,000 to 20,000 miles.

Why Not 3,000 to 5,000 Miles?
If you’re a novice with axles, you may be asking, “Why not change my axles’ gear oil every 3,000 to 5,000 miles like engine oil?” The answer here is multi-fold. Engines have to deal with a lot more heat than gear oil, so the engine oil breaks down quicker. Also, contaminants like soot and dirt particles are much more likely to mix with the engine oil than with the gear oil in a front or rear axle, which is a relatively self-contained environment.

What If I Don’t Change It?
We’d hazard to say that most people neglect their axle oil, which begs the question—what can happen if it’s not changed? Well, several things, and most of them are bad. In humid climates, moisture condensation can form inside the housing, and if the truck isn’t driven often, water contamination can be the result. Water and metal means rust, which isn’t desirable in the least. Also, the antioxidants and detergents used in gear oil can break down, and if the vehicle is run long enough, the oil itself can change viscosity, leading to inadequate lubrication and bearing or ring and pinion death. A repair bill to rebuild a rear axle on your average diesel truck can run in the thousands of dollars, so changing your axles’ gear oil at regular intervals is definitely a good idea.

Synthetics and Multi-Weight Oils
While old-timers may rely on the old standard SAE 90 weight gear oil, newer multi-weight oils, such as Amsoil’s 75W-140 synthetic, maintain their viscosity at temperatures from -31 degrees to 310 degrees. For longevity purposes, however, axle oil should usually not exceed 250 degrees, with the sweet spot being in the 180- to 200-degree range. Another benefit of synthetic oils is that the range between oil changes can be extended (up to 30,000 miles for severe-duty applications, and 50,000 miles if the truck isn’t used for off-roading or towing).

Lubrication Summary
Although gear oil is often neglected, it shouldn’t be. The cost of an axle oil change is very cheap compared to replacing worn-out parts. If it were us, we’d go with a high-quality synthetic from a familiar brand name, watch out for water, mud, or dirt contamination, and change the gear oil every 30,000 miles.

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