- Jun 5, 2002
- Iowegia - USA
White Paper - Synchromesh Manual Transmission Lubricants Since there has been so much discussion and questions about MTL's, here is a white paper that I hope answers some questions.
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. 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. 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 the above percentages of EP additive. The exception of course is ATF fluid used in some of the newer transmissions. Therefore, both lubes contain the same EP additives, just in different strengths or additive ratios. 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. 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. 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. 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 GL4 viscosites are: 1. ATF Series - Type; 6.5 to 8.5 cSt (Equivalent ATF viscosity; Note: ATF additive package is weak compared to most GL 4'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 ( Redline'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, Redline'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 diffy 75W90 (GL5) usually has a higher viscosity than does an mtl in the same advertized weight.