Most GL5 differential and GL4 Manual Transmission oils contain sulfur-phosphorous EP packages. GL4 does NOT refer to any specific viscosity, but it refers to a level of AW/EP protection for the gearing and bearings in a transmission. GL4-rated oils contain about 40% to 60% of the EP additives that GL5 oils contain. Therefore, both MT and Differential lubes contain the same EP additives, just in different strengths or additive ratios.
Both differential and manual transmission fluids use chemical compounds that subdue or inhibit the corrosive effects of sulfur and phosphorous such as calcium, magnesium, boron, potassium or other basic compounds. Emulsifiers, corrosion and rust inhibitors also are included to do their respective jobs.
GL5 differential lubes use friction modifiers to reduce mechanical and fluid friction and add some anti-shudder friction modifier for limited slip, both very different chemical compounds.
GL4 manual transmission fluids use a different friction modifier for synchro engagement, a modifier that does NOT contain the same Friction Modifier chemicals as differential lubes.
Most manual transmission "specific" fluids (GL4) contain about 40% to 60% of the EP additive of differential lubes (GL5) with inactive or buffered sulphurs. GL4 has come to infer a gear lube with these percentages of EP additive. The exception of course is ATF fluid used in some of the newer transmissions. Manual Transmission fluids use a different friction modifier specifically designed for synchro engagement, a modifier that does NOT contain the same chemical compounds as do differential lubes.
Ever since the synchromesh-type fluids appeared on the scene (such as the GM Synchromesh fluid), drivers have had better shifting due to better synchro engagement, attributed to the specialized friction modifier used in these lubes. This specialized friction modifier is better for metallic and composite synchros in terms of shifting and life.
A synchromesh fluid usually refers to a specialized fluid that contains special friction modification additives for transmissions that use mechanical synchronizer assemblies; those synchronizer assemblies may be made of carbon fiber composites, sintered metal, brass/bronze, or steel-steel materials.
You also have to consider the viscosity of the fluid that the transmission was designed for. The spectrum now ranges from ATF to 75W90 viscosities and therefore a synchromesh GL4 Manual Transmission Lubricant (MTL) can be any viscosity from 7.0 cSt (ATF equivalent viscosity) to a 75W90 type viscosity of approx. 14.5 cSt, and contains special friction modification additives for synchronizer assembly engagement, brass/bronze, or steel-steel materials.
Current MTL viscosities are:
1. ATF Series - Type; 6.5 to 8.5 cSt (Equivalent ATF viscosity; Note: ATF additive package is weak compared to most GL4's)
2. Synchromesh Series -Type; 9.3 - 9.5 cSt (such as Amsoils MTF, Texaco's MTL, Pennzoil's Synchromesh, GM and Chrysler's Synchromesh)
3. 75W85 Series-Type; 9.8 to 11.5 cSt ( Red Line's MTL, RP's Synchromax LT, Nissan's MTL, Honda MTL, Castrol Syntorq LT)
4. 75W90 Series-Type; 12.8 to 14.5 cSt (Amsoil's MTG, Red Line's MT-90).
MTL specific lubes we're developed for manual tranny's and transaxles, and not for differentials or industrial gear boxes. A differential lube may not kill your tranny, but it is not the optimum lube for it. A differential 75W90 (GL5) usually has a higher viscosity than does an MTL in the same advertised weight. Synchromesh Manual Transmission lubes were developed specifically for manual transmissions and transaxles, and not for differentials or industrial gear boxes.
Many if not most GL5 fluids rated for differentials may also be rated as MT-1. Some manufacturers, such as Amsoil, state the following:
“Recommended for use in differentials, manual transmissions and other gear applications requiring any of the following specifications: API GL-5, MT-1, MIL-PRF-2105E, Dana SHAES 234 (Formerly Eaton PS-037), Mack GO-J, or the differential (hypoid) gear oil specifications from all domestic and foreign manufacturers such as GM, Ford and Daimler Chrysler. Can also be used in axles where an API GL-4 lubricant is recommended.”
This MT-1 rating refers to use in heavy duty truck transmissions, and as such, does NOT necessarily mean that this fluid will function properly in a light truck or car synchromesh transmission.
