05/15/2026
Understanding Differential Gear Oil and why its NOT RECOMMENDED to mix different types and weights......no gaslighting or bu****it, all information in this write up is based on real world experiences and technical data from major lubricant manufacturers!
It's a long read, so buckle up
Let's begin with an overview-
Much like how engine oil provides a protective layer on moving engine components, differential fluid, often simply referred to as gear oil, does much the same thing for your vehicle’s differential components.
While your engine uses an oil pump to provide pressurized oil to the engine’s bearings, lifters, etc. the differential uses “splash” oiling by way of the movement of the ring and pinion gears slinging the oil throughout the differential housing.
This differential fluid is used to lubricate the gears, differential and axle bearings, and other components within the axle housing. This gear oil coating also provides heat dissipation of the heat generated by the friction of the ring and pinion gear contact (in racing environments some vehicles use a cooler and pump to circulate the gear oil to an external cooler and then back to the housing).
The gear oil also acts as a corrosion inhibitor, acting as a protective oil film coating on steel and cast iron components inside the differential and axle housing.
What is gear oil weight?
A gear oil’s weight, or viscosity, is measured like engine oil by its thickness with a numerical value associated with it—albeit much thicker.
Where engine oil may be in the 10W30 range for a multi-weight oil, gear oil is more often in the 80w90 or 75W90 range.
Like engine oils, the first number shows the oil’s thickness when cold, while the second number denotes the thickness value once the oil is at operating temperature.
Viscosity is the oil’s resistance to flow. Using the correct viscosity oil (and any differential oil additives) is critical for proper operation of your differentials internal components, as well as the ring and pinion gears themselves.
Higher viscosities suit extreme usage needs but can be too thick for some differentials to work properly.
It is always best to use what the differential manufacturer recommends in most cases for optimum torque transfer and axle oil temperature, while usage and climate will sometimes provide slightly different viscosity needs.
And, just like specialty engine oils, you can find gear oil in single weight viscosity offerings as well.
Mixing different weights or brands of gear oil is generally a bad practice, as it can compromise the performance and protection of the gear assembly (too low a viscosity, not enough friction modifier, etc.).
Mixing additive packages, viscosity ratings, and even base oils are all conditions that may lead to reduced effectiveness of the gear oil’s capabilities.
Topping off your differential with an unknown oil (or not knowing what gear oil is in the differential) is not recommended.
So instead of asking “Can I use 80W-90 instead of 75W-90?” to top off your differential, it is best to drain the differential and refill with the correct amount and type of fluid with the proper additive package your differential requires.
Can Synthetics MIXED With Other Oils?!
What are the consequences and/or benefits of mixing synthetic and non-synthetic oil in an engine crankcase, or any other application such as a differential?
Maybe you have been told that an axle housing can be drained of non-synthetic oil and synthetic oil can be added without flushing any of the prior oil residue.....
In general it is never a good idea to mix two different oils in any application.
If this is unavoidable, there are two major concerns, specifically incompatibility between the base oil, and incompatibility between the additives.
The overwhelming majority of synthetic oils, particularly crankcase oils are PAO based.
From a base oil standpoint, there are usually no problems with mixing a PAO synthetic and a mineral oil since both are hydrocarbon based and chemically very similar.
However, extreme caution should be used if the synthetic oil is non-hydrocarbon based.
As for additive incompatibility, the biggest danger is mixing oils with very different additive packages, particularly mixing those with acidic additives, such as most AW and EP oils, with oils containing alkaline additives commonly used in crankcase oils. In this case, mixing can result in a number of problems including additive drop out, loss of demulsibility, foaming problems and reduced oxidation stability.
The best approach is to select the appropriate lubricant for the application at hand and stick with it! If you are forced to mix oil of different types, it is important that either the lubricant manufacturer is consulted and/or compatibility testing is performed to avoid potential problems.
There are four main families of gearbox oils:
• Mineral oils
• Poly-alpha-olefins (PAOs)
• Esters
• Polyglycols, also referred to as Poly Alkylene Glycols (PG or PAG)
Mineral oils, PAOs and esters may all be mixed together.
Only polyglycol type synthetic oils (PG or PAG) may not be mixed with the other lubricant types. They can react together and form gum gels in the casings.
Furthermore, they can sometimes be incompatible and immiscible with other polyglycols.
Basic principle is that your gears never want to get below a floor viscosity, a minimum hot. What happens cold stays in Vegas. Its always too thick cold no matter what you use.
That said, the higher the VI spread, the more the viscosity will stay thicker at higher and higher temperatures.
As long as you do not mix two different synthetic bases you are good, there is a chance that two different synthetics will react and drop solids but if you just added 100% mineral to a synthetic there is no issue whatsoever.
This is also why when switching between synthetics and conventional oils, it is recommended to run which ever type you want to use for a few days between to flush/dilute any of the original left behind, even if it takes a couple of drain and refills.
Better to be safe than sorry!!!!
If you read this far, you are now equipped with some knowledge to better help understand gear oil function and application, and how best to select your gear oil when the time comes.
Thanks!!!
