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Differences Between Gear Oil Weight? (75W-90 | 75W-140)

Created by Taylor Ward
Last Updated 3/6/2020

Many of our customers often wonder what the differences are between our 75W90 and 75W140 gear oil kits, so we at LMR have provided an in-depth look at all of the detailed aspects.

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Differences Between Gear Oil Weight? (75W-90 | 75W-140) - Differences Between Gear Oil Weight? (75W-90 | 75W-140)

Why Do We Offer Different Weights For All Of The Different Years?

Depending on the climate where the customer lives can highly determine the overall thickness of the oil at freezing and at 100 degrees. We put these kits together with oils that can really work on both sides and not have issues in most cases. For example, if you take the 86-09 ring and pinion kits, we include the 75W90. We consider this a more “universal” all year weight over some of the 100% specific weights these cars call for.

Breakdown Of What The Numbers Mean

Differences Between Gear Oil Weight? (75W-90 | 75W-140) - Differences Between Gear Oil Weight? (75W-90 | 75W-140)

To break it down further the first number refers to the maximum temp at which the gear oil will reach 150,000 CP (centipoise). Centipoise is a measurement of dynamic viscosity that is measured in the centimeter gram second (CGS) system of units. This is a multiple of the CGS base viscosity unit that is named poise (P).

The “W” that is located in the middle simply stands for “winter”, which means that it has been tested in a colder temperature than normal. Gear oils are normally tested around 100 degrees Celsius.

The second half of each number, whether it be the 90 or 140 simply refers to the viscosity at 210 degrees Fahrenheit. The 90 being between 14 cSt (centiStokes) and 25 cSt and 140 being between 25 cSt and 43 cSt at 210 degrees Fahrenheit. Centistoke is a decimal fraction of the CGS unit of kinematic viscosity stokes that is equal to a centimeter per second (cm²/s).

Check out our other article for more information on the breakdown of the different Mustang Fluid Capacities!


The first number refers to a cold-weather viscosity. The lower this number is, the less viscous your oil will be at low temps. The “W” stands for “winter” in contrast to what others think that it means “weight” which it doesn’t. Lastly, the second half of each number refers to hot weather viscosity (how fluid your oil is at hot temps). Basically, the higher the number is, the thicker the oil is at a specified temp.

We at LMR hope that this in-depth breakdown will better help you in making your decision about which type of oil best suits your needs.

Mustang Gear Oil & Friction Modifier

About the Video

How To Change Mustang Rear End Gear Oil - LMR Basics

Check out our YouTube channel for even more tech tips, installation videos, how-tos, and more. The best place to go for anything Mustang related!

Published on 2016-12-02
In this installment of LMR Basics, check out the simple steps required to change the rear end gear oil in your Mustang.

Whether you're changing rear end gears or doing routine maintenance, this video will show you the general procedure on changing the rear end oil in your Mustang.

Shop LMR.com for the best selection of rear end parts for your Mustang.

What’s up everybody! Landan with Late Model Restoration. In this installment of LMR Basics, I’m going to be bleeding the brakes on this 1992 Fox Coupe.

Every once in a while the rear end in your Mustang needs servicing. It plays a vital role in vehicle performance because after all its job is to transfer power from the engine to the rear wheels. This task is very simple, and often overlooked by many because of the intimidations from rear end components.

Before changing the rear end gear oil in your Mustang you are going to need a few supplies to get the job done. You will need a lift or jack and jack stands, a socket set, flat head screwdriver quality brake clean, rags or shop towels, a putty knife or gasket scraper, a quality liquid gasket product or one of our Lube Locker reusable gaskets, a drain bucket, the correct rear end gear oil weight and friction modifier, and lastly; a clear tube to transfer the fresh gear oil into the housing. The alternative to this is to purchase a transfer pump from your local auto parts store.

To begin, support the vehicle via a lift or jack stands. Position the drain pan underneath the rear end housing. Loosen and remove all of the cover bolts except for the top one. Leave this tight and in place so you can pry on the cover.

Now, use a flat head screwdriver to pry open the cover so the old fluid can drain out. Once the majority of the fluid has ran out, loosen and remove the top bolt. Finally, remove the cover and set it aside.

Use this time to scrape away any old gasket maker and thoroughly clean the rear end with quality brake clean. Rotate the carrier to ensure all surfaces are clean. Do the same for the differential cover.

Once complete, scuff the rear end housing surface and diff cover with a Scotch-brite pad to further clean the mating surfaces. I used a Lube Locker gasket from our site, LMR.com to ensure a proper seal between the cover and housing. Option two would be to use a quality gasket maker and apply a thin bead around the differential cover.

Reinstall the cover and then torque the bolts to twenty to twenty-five pound feet, working in a star pattern.

Now, remove the fill plug with a three eights ratchet and three inch extension. Use some clear tube to fill the rear end or the transfer pump. I personally squeeze a little fluid in and then make a hole in the bottle to pressure it with an air gun. It’s one of those things, if it works, then stick with it.

Make sure and use the correct gear oil and the right amount of gear oil. Pretty much all eight point eight rear ends take right at two quarts of gear oil.

Once the rear end is filled, tighten the fill plug and then double check your work. That completes a successful rear end gear oil change.

As far as a time frame goes, if you take your time you can probably bet on about an hour and a half to two hours. A couple things to keep in mind when you changing gear oil; this is a great time to replace axle bearings and seals, rear end gears, and service the carrier if needed. For the correct rear end gear oil change interval, be sure and reference your owner’s manual.

Until next time guys, subscribe to our YouTube channel if you haven’t already done so; be sure and check out our wide selection of rear end parts and accessories for your Mustang from the real Mustang enthusiasts, LMR.com!