The Great Lube Hunt

The Oil You Choose for your Manual Transmission Might Just Save its Life

Words: Jeff Smith

Not all that long ago, a typical service station would stock one gear lube for any manual transmission and rear axle assembly. Perhaps you needed an additive for a limited slip application but that was about as complex as it got. But with late model cars comes a bit more complexity.

The manual transmission landscape has changed dramatically in the last two decades and the gear lube requirements have evolved right along with them. The scary part is – what is it that you don’t know? Choose the wrong gear lube for your manual trans and you might be faced with a minor case of poor shift quality. Or, it might be bad enough to damage synchros. Much of this can be avoided by a short lesson in gear lube technology. We’re going to concentrate here only on domestic performance manual transmissions because this story is confusing enough without clouding the issue with automatics and rear axle assemblies. After researching nearly all domestic performance manual transmissions, it appears that there are three different fluids, four if we count the odd requirements of early non-synchro manual transmissions and the non-World Class T-5 five-speeds that essentially used motor oil as their lube.

Turning our focus to the most popular modern manual transmissions, we’ve found that these gearboxes demand three different fluids. The first and oldest of the three are the four-speed manual transmissions used in domestic performance cars from the late ‘50s through the ‘80’s. These manual gearboxes have been dubbed “yellow metal” transmissions in reference to their use of brass/bronze/copper alloy synchronizer rings. As mentioned earlier, these transmissions used the same gear lube intended for rear axle assemblies. However, in 1995 the older gear lube standards GL-1, GL-2, GL-3, and GL-6 were rendered inactive by the Society of Automotive Engineers (SAE) and updated to the current GL-5 gear lube standard.

This GL-5 standard increased the concentration of extreme pressure (EP) additives (basically sulfur) in the gear lube and initially everyone assumed this gear oil was considered safe to back-date to GL-4-rated manual transmissions. However, tests have shown the GL-5’s sulfur content to be harmful to the soft metals and will, especially at higher operating temperatures, contribute to increased synchronizer wear. According to a quote in Machinery Lubrication magazine “EP additives that contain sulfur cause the most damage to these (yellow) types of metals.”

So this means that all these older manual transmissions like the GM Saginaw, Muncie, Borg-Warner T-10, and Super T-10, Ford Top Loader, Chrysler New Process, and others should only be filled with GL-4 spec fluid. The issue is that this fluid is becoming more difficult to locate, especially at your local auto parts store. Fortunately, we found several gear lubes that do fit the GL-4 requirements like Sta-Lube’s GL-4 rated 85W-90 gear oil and Amsoil’s synthetic GL-4. These fluids still might be difficult to find if needed immediately, but can be purchased online.

We’ve seen statements made by individual “yellow metal” transmission builders contending they have not witnessed any ill effects from using GL-5 lubricants in their transmissions. This type of anecdotal information can be found online and is typical of the information floating around the internet in countless forums. There will probably be certain, low-load situations where synchronizer damage may be minimal but the fact remains that GL-5 lubricants have increased concentrations of sulfur and phosphorous (replacing zinc and phosphate – ZDDP) to achieve improved EP ratings.

According to Lubrication Magazine, there are both active and inactive types of sulfur additives used in most EP additive packages. The most aggressive is the active sulfur which can quickly corrode especially when gear oil temperature increases during aggressive driving like autocrossing or road course track sessions. So while some yellow metal trans builders suggest that a GL-5 lubricant can be used in a GL-4 intended transmission, the chemistry tells a different story.

We also spoke with Paul Kostelnik of Liberty Gears to get that company’s perspective on this gear lube question. Paul reinforced the concept of using strictly a GL-4 lubricant for any yellow metal transmission such as the Muncie, Top Loader, or Chrysler four-speeds. He also suggested avoiding the use of any 80W90 synthetic because, he says, the molecules are shaped differently which reduces the necessary coefficient of friction between the synchros – causing them to create an undesirable clash on the gear change.

We have some personal experience in this situation with a Richmond Super T-10 transmission in which we tested a synthetic 70W80 gear lube in an attempt to reduce the viscosity. The net effect was exactly what Kostelnik described with excessive gear clash on the gear change no matter how gently we moved the shifter. We immediately drained the fluid, replacing it with a GL-4 and the problem was solved.

Kostelnik also emphasized that gear lube temperatures should never exceed 275 degrees F. If this temperature threshold is exceeded, the gear lube should be replaced with fresh fluid because higher temperatures have a significant negative effect on the lubrication properties of a petroleum-based GL-4 gear lube that will cause major damage if not replaced.

Just to be clear, this is not to infer that GL-5 is a poor lubricant. On the contrary, the increased EP additive package in GL-5 lubes is exactly what is necessary to create a high performance lubricant for hypoid gear rear axle applications. As long as it’s used in its proper orientation for rear axles and hypoid gear applications, GL-5 lube is what you should use.

Now we can move on to the next big evolutionary change in gear lube that occurred with the creation of the original T-5 trans in Mustangs in 1983. These transmissions predated the World Class T5’s that first appeared in the 1985 5.0 L Mustangs. The T-5 continued to be used all the way up to 2010 in V6 Mustangs. For GM cars, the 5.0L Camaro entered the fray in roughly 1990. In these cases, the T-5 and later the T-56 six-speed manual gearbox specified the GM Dexron III ATF fluid. This was done for two main reasons. The thinner viscosity is especially useful in low temperature applications while it also created the proper coefficient of friction necessary for the redesigned synchronizers. These new-generation synchros employed graphite composite and cellulosic/composite materials that demanded this type of fluid.

The original fluid spec for these transmissions was the then-current automatic transmission fluid known as GM’s Dexron III or Ford’s version Mercon (eventually Mercon V). But it didn’t take long for the fluids to evolve. GM eventually discontinued its Dexron III in favor of what is now Dexron VI in 2006 model year cars. For nearly all GM automatic transmissions, Dexron VI is backward compatible – meaning that it will work in older automatics. But that was not the case for the older manual transmissions.  GM even published a Technical Service Bulletin warning that Dexron VI is not backward compatible with these manual transmissions and that only a fluid equal to Dexron III should be used. The latest GM version of the previous Dexron III is now called Manual Transmission and Transfer Case Fluid (PN 88861800) that we found on Amazon for around $11.00 per quart.

But this is not the only fluid available. We found several aftermarket companies selling their version of Dexron III ATF including Amsoil, Mobil1, Pennzoil, and Royal Purple among others. Some of these fluids are synthetics, which Tremec says to avoid using in their transmissions. Tremec claims that “Some brands of synthetic fluid contain powerful detergents and additives that can prove harmful to your transmission’s synchronizers.” They also say that use of synthetic gear lubes will void the warranty. It’s interesting that at least from a viscosity standpoint, our chart indicates that there is no difference in viscosity between Mobile’s synthetic, synthetic blend, and the conventional ATF. But this does not take into account the additive packages that include friction modifiers.

The waters really get muddy with newer cars as it seems now that you need a catalog the size of the New York City phone book just to cross-reference the vast array of manual transmissions and their specific gear oils. This makes it impractical to list all the different transmissions and their recommended fluids, so the best thing we can offer is to do your own research or perhaps call one of the gear lube companies directly. Our Source list contains the technical hot line numbers for several of the major oil companies.

As an example of how confusing all this can be, we’ve found seven different GM lube oil part numbers just for late model GM manual, automatic, and rear axle applications. There are probably more but we gave up because it became far too confusing to try and make sense of it all.

The third fluid on our manual trans lube hit parade is very similar to ATF – a fluid first referred to as Synchromesh by GM. This is actually a slightly more viscous fluid compared to ATF as evidenced by the viscosity ratings for Synchromesh as listed in the accompanying chart. Synchromesh can be defined as an ATF-like fluid that contains special friction modifiers that are intended for specific manual transmissions with carbon fiber, sintered metal, and composite synchronizers. As an example, Tremec recommends Synchromesh for its TKO-500 and 600 model overdrive five-speed transmissions.

We’ve also seen several mentions on forums from enthusiasts concocting their own witches’ brew by mixing different fluids together into a custom blend. Lake Speed, Jr. of Driven Racing Oil says this is a dangerous game, especially if the two fluids are not from the same company. But even mixing fluids from the same company is not recommended since the blending of the additive packages may do more harm than good.

It’s beyond the scope of this story to go much more specific than this overview because of the vehicle and year-specific requirements that occur with later model vehicles. So we’ll leave that part of the investigation up to you, should you desire to dig a little deeper. For those enthusiasts buying a used transmission, it would certainly be in your best interest to know the transmission’s exact year and application so that you can determine what gear lube it requires. In today’s age of complexity, guessing the transmission’s gear oil requirements is a poor way to row through the gears.

Transmission Oil and Gear Lube Chart

In this chart, we have listed as many oils and their specifications as possible. The column entitled Viscosity at 212 degrees F (100 degrees C) is important because this is a normal manual trans oil temperature. From these numbers you can see how different lubes within the same family (GL-4, for example) offer drastically different viscosity ratings. A higher number means the oil will be more viscous (“thicker”) at that temperature. That may help – or hinder – synchro performance. This chart also offers the opportunity to compare viscosities and perhaps offer direction toward fine-tuning your transmission’s shift characteristics. The Viscosity at 212 degrees F is not, however, an accurate indication of how viscous the fluid will be at lower temperatures, which is an important consideration for cold weather operation.

The second rating is the lubricant’s Viscosity Index. This number relates to the viscosity’s consistency over its entire temperature range. A higher number can be considered better although there are qualifiers to that statement that would require another story to fully explain. A generic oil viscosity curve follows conventional wisdom with thicker viscosity when cold and thinner as its temperature increases. A higher Viscosity Index number indicates the fluid maintains better control over this range of viscosity. Evaluating these two performance characteristics may give you a better idea of how these lubricants perform as opposed to relying solely on marketing explanations.

Transmission Oil Viscosity @ 212 Deg. F Viscosity Index PriceQt.
GL-4 Gear Lubes
Red Line MT-80, 75W80, pn 50204 10.4 185 $16.95
Red Line MT-85, 75W85, pn 50504 12.0 186 $16.95
Amsoil Man. Trans 75w90 Synthetic 13.9 N/A $15.75
Brad Penn Classic Synthetic 80w90 15.0 102 $10.18
Brad Penn Classic 75w90 pn 7729 N/A N/A $10.18
Royal Purple MaxGear* 75w90, PN 01300 17.5 N/A $14.94
Royal Purple MaxGear 75w140, PN 01301 28.8 N/A $17.78
Sta-Lube GL-4, SAE 85w90  pn 24229 N/A  N/A $14.09
Synchromesh Trans/Gear Lubes
Royal Purple Synchromax pn 01512 7.7 175 $15.32
Valvoline Synchromesh pn 811095 8.8 157 $  8.99
Driven STF Synchromesh, pn STF 8.9 162 $19.99
Pennzoil Synchromesh pn 3501 9.1 208 $  8.97
Amsoil Synchromesh 5W30 pn  MTFQT 9.7 190 $13.65
ATF Gear Lubes
Lucas Multi-Vehicle ATF, pn 10418 7.3 159 $  7.97
Mobil Multi-Vehicle Synthetic blend ATF blue 7.4 193 $  6.00
Mobil ATF D/M, pn 113126 7.4 183 $  6.70
Valvoline ATF, Dexron III, pn VV360 7.4 N/A $  6.70
Amsoil Multi-Vehicle ATF, pn OTFQT-EA 7.5 155 $  9.45
Amsoil Synthetic ATF, pn ATFQT-EA 7.5 166 $12.45
Royal Purple Dex III, pn 01320 7.5 180 $12.79
Red Line D4 ATF, pn 30504 7.5 186 $11.95
Pennzoil Multi-Veh. ATF Dex. IIIH, 550041916 7.6 204 $  9.97
Pennzoil Dexron III, Synthetic  pn 550041916 8.38 N/A $  9.97
  • Royal Purple’s MaxGear is rated GL-4 and GL-5 but claims this gear oil is “Non-corrosive to yellow metals”. This is the only traditional gear oil listed here with GL-5 specs.



Driven Racing Oil


Liberty’s High Performance Products

Lucas Oil Products

Brad Penn Grade 1 Oil

Modern Driveline

Pennzoil Products

Red Line

Royal Purple



About Jeff Smith

A clue into how long Jeff Smith has been writing technical automotive stories might be his following of second generation readers. Writing continuously for nearly 40 years, his focus with Xceleration covers all things technical. His collection of cars includes a bevy of Chevelles and El Caminos. When not writing about cars, he likes to spend time with his wife Valerye, children Amber and Graham, and granddaughter Celeste.