What’s The Best Gear Oil For The 14 Bolt Axle?

Wondering which gear oil is best for your 14 bolt axle? You’re not alone – it’s a common question, and the answers you’ll find online are all over the map. Many people recommend 80W-90 or 75W-90, but we have found that the best gear oil for the 14 bolt axle is 75W-140.

We say this because we work closely with 14 bolt axles by virtue of selling so many disc brake conversion kits for them. We consider ourselves pretty well-versed in all things 14 bolt, and we recommend synthetic 75W-140 gear oil for the 14 bolt without hesitation.

We’ll explain why we like this gear oil the best, and also address some common questions that people have about gear oil and the 14 bolt axle.

First, Here’s Why Gear Oil Is Important

Image Credit: ChrisFix

Gear oil serves three main functions:

  1. Lubrication and protection from wear
  2. Cooling
  3. Corrosion protection

To do these three things well, gear oil needs to be stable. Stable gear oils resist foaming (which reduces lubrication ), and also don’t breakdown when exposed to heat.

Is Synthetic Gear Oil Really Best?

Just like engine oil, gear oil is available in both conventional (aka “dino oil”) and synthetic formulations. And, just like engine oil, there’s a pretty good argument about the cost/benefit of synthetic oil vs. less expensive conventional oils.

Generally speaking, the key difference between synthetic and conventional oil is molecular consistency:

  • In conventional oil, the individual molecules that make up the oil are in a lot of different sizes. This is because conventional oil is always a little bit of a blend of oils.
  • Synthetic oil is almost perfectly uniform on a molecular level. It’s basically just one oil.

When you have a blend of oils (conventional), you have inconsistent molecular breakdown. Conventional oils can start to breakdown within 15 minutes of use, because at least some of the molecules in the blend aren’t a match for the intended use. Broken oil molecules accelerate breakdown, because they basically act as a solvent. Usually, conventional oil is completely broken down much sooner than synthetic oil.

Synthetic oil, on the other hand, will last a very long time without breaking down. It’s just one molecule, and that molecule is perfect for the application. Breakdown is far less likely with a synthetic oil.

Because synthetic is very resistant to breakdown, most new vehicles use synthetic gear oil (including new 14 bolt axles). Synthetic oil also resists foaming, which makes it the best choice for heavy duty applications.

Of course, it’s not as if conventional oil is bad. While some of the molecules in conventional oil will break down right away, most of them will last long enough to do the job. What’s more, because conventional oil is sort of a mix of oils, it tends to bond pretty well with additives. These additives can help make up for any breakdown that takes place.

Still, conventional oil is a mix of different oils. It’s not quite as good as full synthetic.

Why Use 75W-140 Gear Oil?

75W-140 gear oil is a good option for a wide range of temperatures. Unlike 80W-90 or 75W-90, 75W-140 offers protection at both colder temps (75) AND fairly high temps (140). While gear oil shouldn’t spend a lot of time at very high temperatures, it’s good to have protection in those scenarios. Likewise, you want protection at the lowest temperature possible for those few minutes when everything is still cold after start-up.

The downside to using 75W-140 gear oil is the cost. It’s about twice as expensive as conventional 80W-90, and maybe 30% more than conventional 85W-140 gear oil.

Still, the absolute cost difference is typically less than $20. For $20, it’s kind of a no-brainer.

How Much Gear Oil Does A 14 Bolt Axle Need?

The gear oil capacity of a 14 bolt axle is either 3.6 quarts (full float) or 2.75 quarts (semi-float). However, when it’s time to fill the axle, don’t try and measure out the fluid. Instead, just fill the differential slowly, and stop pumping gear oil in when fluid comes out the fill hole.

NOTE: Some aftermarket differential covers increase the fluid capacity of the axle. While more gear oil isn’t automatically better (it may or may not be – depends on the design of the cover), it’s important to note that our capacity numbers are for a 14 bolt with a standard OEM cover. If you have an aftermarket diff cover, be sure to check the capacity accordingly.

How Often Should I Replace The Gear Oil In My 14 Bolt Axle?

New fluid

For many new passenger cars, the gear oil added to the differential at the factory will last the vehicle’s lifetime. But for heavy-duty trucks with heavy-duty uses, it’s a good idea to drain and replace gear oil regularly.

Generally speaking, gear oil should be replaced every 15-30k miles, or anytime the axle is submerged underwater. This ensures that the oil provides maximum protection to the differential gears.

Summing Up

To our knowledge, no one has ever regretted using the best oil possible to protect their differential.

While it’s always a good idea to question the value of a premium product like synthetic gear oil, we believe it’s the best choice for heavy duty axles. Synthetic oil doesn’t breakdown the same way that conventional oil does, and 75W-140 provides the widest possible range of protection for your differential.

Considering synthetic oil is usually only a few dollars more than conventional, it’s the logical choice for anyone concerned about protecting their 14 bolt.