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.
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.