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Gear Oil 101: Which Lube is Right for Your Manual Transmission?

Tremec Transmission

Selecting the right gear oil or transmission fluid is critical to the life and performance of your manual transmission. (Image/Tremec)

Similar to what we discussed in our Automatic Transmission Fluid 101 post, selecting the right gear oil for your manual transmission isn’t as easy as it might seem.

There are dozens of different types of gear oils, each with its own special designation and use, so we created this handy primer (oil pun!) to help you make good decisions.

Keep in mind that most modern manual transmissions have gear oil service intervals well north of 50,000 miles, with many being “lifetime” oils.

Driving enthusiasts, however, change gear oil to deliver improved transmission performance—like smoother shifting or added durability in extreme conditions. If this sounds like you, keep reading.

Gear oil, AKA gear lube, is often used in your manual transmission’s gearbox, and you’ll commonly find it in older transfer and differential cases too. But gear lube isn’t the only choice for a manual transmission. In fact, many modern manual transmissions actually spec for automatic transmission fluid instead of a traditional gear oil. Regardless of what your manual transmission uses, the fluid or oil’s primary function is lubrication—preventing metal-to-metal contact between the meshing gears.

Gear oil is different from engine oil.

For starters, gear oil can come in much higher viscosity ratings. In other words, a 80w-90 gear oil is much “thicker” than a 5w-30 engine oil.

But viscosity is only part of the equation. Here’s why:

Manual transmissions are often made up of different metals. The gears can be made of a hardened steel, while the transmission’s synchronizers (AKA syncros) are often made of a softer metal, like brass.

What’s good for one metal may adversely affect the other—so companies had to develop formulations that offered the requisite lubrication, yet wouldn’t harm any of the transmission’s components.

This is also where GL ratings come into play.

The most common ones you’ll see are GL-4 and GL-5. Those ratings basically reflect the oil’s ability to function effectively given different driving conditions (read: pressure between meshing gear teeth). GL-4 oils are typically found in most daily driven cars. while GL-5 oils are often reserved for high-stress, high performance applications like trucks and high-powered cars.

Your owner’s manual will tell you exactly what GL rating your transmission requires.

Sometimes you’ll see a bottle labeled MT-1. It’s intended for non-synchronized transmissions, like those found in heavy-duty commercial trucks, and certain four-wheel drive transfer cases.

Red Line Gear Oil

You can find the gear oil’s viscosity and GL rating on its label. (Image/Summit Racing)

What about transaxles?

Though the role of the gear oil is the same, there are extra factors at play in a transaxle. That’s because transaxles function as both an axle and transmission. You can read more about transaxles and how they differ from transmissions here.

For instance, you might find different oil recommendations depending on whether the transaxle has a built-in limited-slip differential (LSD). In other words, seemingly identical cars may require different oil, depending on the presence of an optional LSD.

So, what should I use, then?

We could dedicate a dozen posts to the answer, but since this is a 101, here’s the simplest solution: Check your vehicle owner’s manual.

It will spell out exactly what oil you should use, complete with viscosity and GL values specific to your vehicle.

Bottom line: To avoid transmission damage, you must consider several factors (including viscosity and formulation) when selecting your gear oil.

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10 Comments

  1. Pingback: Transmission Fluid 101: Which ATF is Right for Your Vehicle? - OnAllCylinders

  2. mine 99’5-speed ram 1500 exploded after using 75-90 believing it wasn’t wright after just rebuilt less than 6mos.

  3. William Ottney says:

    Running Red Line MT90 in my TKO600. I found that the Manufacturer-recommended lubricant was too low viscosity in very hot weather. After Power Tour 17, I changed to Red Line and found a huge improvement in hot weather.

  4. Mario Martinez says:

    1991 Chevy Silverado 4sp. What transmission oil should I use. Aluminum case?

  5. Pingback: How Often Should You Change Transmission Fluid? A Guide to Intervals

  6. Pingback: Vehicle Wiring Harness Design - free wiring diagrams

  7. Some owner’s manuals are better than others and they don’t always clarify ‘exactly’ the right fluid to use. My manual says 75W-90 and either GL-4 or GL-5, yet my dealer service department routinely uses synthetic 80W-90 in bulk form and no, they don’t sell it to the public. Then there’s the aftermarket products, which can be similarly misleading. One well-known gear oil manufacturer simply lists it’s product as high performance gear oil on the front label, then states in small print on the back label that it is suitable only for non-synchronized transmissions…which rules out most modern auto transmissions. As with anything, when it comes to automotive info don’t believe everything you hear, and buyer beware.

  8. Pingback: How Often Should You Change Transmission Fluid? A Guide to Intervals - www.dabbledomain.com

  9. Hi I have a t355 manual transmission out of a Jeep compas rebuilding it 5 speed the automatic transmission fluid isn’t doing the job not heavy enough and who would put sealed bearings in a oil bath unit should I leave it that way or remove the seals and put a heavyer oil in

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