Posting it here since people new to BITOG do not look in the White Papers forum. Manual Transmission (MT) Lubricants (Updated 2/9/2017) Ever since dedicated MT fluids appeared on the scene (such as the GM Synchromesh series of fluids), drivers have seen better shifting due to better synchro engagement and improved shifter-fork movement, attributed to the specialized additive package used in these lubes. Before these fluids were introduced, engine oil’s such as 5W30 or gear lubes such as 75W90’s were specified, depending on design. Note: This white paper is directed toward light truck and passenger vehicle manual transmissions. We will not be discussing transmission lubricants for Heavy Duty truck transmissions or those transmissions requiring MT-1 rated lubricants. One of the first of these dedicated MT fluids were the GM Synchromesh series of fluids in the viscosity range of API 5W30 engine oils, or the SAE Gear Oil 80W85 range, or 10.5 [email protected] These fluids were originally targeted for the GM series of synchromesh transmissions. Purported to have been formulated by Texaco, these early MT dedicated GM fluids were partial synthetic fluids that contained a synthetic oil component of alkylated benzene for low temperature operation. The AW component was primarily a reduced level of EP additives with a low level treatment of ZDDP for anti-oxidant purposes. Later, the EP additive was dropped for increased levels of ZDDP as the primary AW additive. Today’s MT fluids contain a multi-functional phosphorus chemistry as the primary AW additive with increased levels of synthetic fluids. Then along came Pennzoil which introduced “Pennzoil Synchromesh” MTF with a 9.3 [email protected] viscosity. This fluid fulfilled the specifications for both GM and Chrysler manual transmissions as their factory fill. Later, companies such as Amsoil and Redline introduced MTF’s in various viscosities. These fluids were successful because they introduced a specific friction modifier chemistry that insured smooth synchro engagement and disengagement. Dedicated or Application Specific MTF’s for Manual Transmission or Transaxle use an additive package containing Anti-Wear (AW) additives, rust and corrosion inhibitors, emulsifiers, and specialized Friction Modifiers, and are generally protection rated at the SAE GL-4 level. Now GL-4 does not necessarily refer to MTF’s, as there are some gear lubes in the Market place that are GL-4 rated, but are not MTF’s. GL-4 is an SAE wear protection rating. There are a few GL-5 rated MT fluids for specific vehicles that have common MT and Differential sumps. One such manufacture, Subaru, has transmissions which shares a common sump and require a MTF which is GL-5 rated. Manual Transmission fluids use a different Friction Modifier for synchro engagement, a modifier that is NOT the same Friction Modifier chemistry as used in differential lubes, engine oils, or hydraulic fluids. Here, Friction Modifier or Friction Modification does NOT refer to friction reduction. Here, Friction Modifier refers to a chemical compound that gives rise to a situation such that the coefficient of friction (COF) varies Dynamically with respect to the relative speed and force of parts that mesh and de-mesh during engagement and disengagement. (For a similar discussion of ATF’s and friction modification, please see: https://bobistheoilguy.com/forums/ubbthr...ion#Post1111352 ). When selecting a replacement fluid for a manual transmission or transaxle, one has to consider the viscosity of the fluid for which the manual transmission or Transaxle was designed. The spectrum of viscosities for light truck and passenger vehicle transmissions now range from 6.0 [email protected] to 75W90 viscosities and therefore a dedicated MTF can be any viscosity from 6.0 cSt with an approximate SAE 70-75W80 grade ( a viscosity or grade similar to ATF’s) to an equivalent SAE 75W90 grade viscosity of approx. 14.5 [email protected] For example, if your transmission requires a [email protected] fluid, an SAE 75W90 fluid should be used. In some cases, a 75W85 synthetic fluid has been shown to provide better cold weather shifting, while still providing sufficient anti-wear protection and fluid film thickness. In many cases, one has to experiment with fluids of slightly different viscosities to determine which fluid works best in your local climate and specific transmission. We should also mention that transmission wear can also require a change in viscosity as well. However, no OTC additive or change in viscosity will cure a transmission that has severe wear in its bearings, gear teeth, or synchronizer assemblies. It is recommended that an examination of clutch engagement and a change in transmission fluid be done before rebuilding a transmission. For the latter, crunching and “notchyness” can be caused by the old fluid having been sheared (loss of viscosity) and the degradation of its additive chemistry. Rather recently, many MT manufacturers have introduced fluid specifications for fluids with lower viscosities in the range of [email protected] to [email protected], a viscosity in the same range as ATF fluids. These fluids were specified primarily to increase fuel mileage in order to raise fleet fuel mileage figures. Some fluid manufactures’ such as Royal Purple and the fluid supplier for GM, have developed MTF fluids in this viscosity range. These fluids are basically ATF fluids with a bump in the AW components. One manufacturer, Tremec for example, currently specifies Dexron III/Mercon ATF fluids. It is the opinion of this author that the design of this transmission can only use ATF because of cold temperature operation problems. In earlier specifications, they specified higher viscosity MTF’s, while in the latest specification, they specify ATF’s, yet the internals are supposedly the same. My guess is they found that, with their original specs for the higher viscosity fluids, shifting was poor in cold weather, so they dropped the viscosity and at that point in time, the only available fluid with a low viscosity was ATF - which is no longer the case. Now there are new low viscosity fluids on the market with improved additive packages. In terms of the composite materials used in the ring-blocker assembly of these transmissions, the friction modifiers in ATF’s do nothing for synchronizer operation. The shearing of the fluid at the interface is the primary means of dynamic friction modification. Current MTF’s in the range of 8 cSt to 10 cSt, designed for metal-alloy synchronizer assemblies, have been shown to work in these same transmissions without blocker-ring degradation or excessive wear. One last comment: MTF specific lubes we're developed for manual transmissions and transaxles, and not for differentials or industrial gear boxes, and vice versa. A differential lubricant is not a good choice for MT’s. A dedicated differential fluid of 75W90 with a GL5 rating usually has a higher viscosity than does an MTF in the same advertised grade, and will therefore cause shifting problems in cold weather. In addition, a differential lube does not contain the proper additive package needed for MTF’s, since it contains an Extreme Pressure additive package meant for highly loaded hypoid gearing. Feel free to comment, ask questions, or to discuss new MT fluids.