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4. What is the difference between Gearbox Oil and “Manual Transmission Fluid”?
Generally speaking, it has long been the case that the liquid used in automatic gearboxes has been referred to as transmission fluid, but the liquids used in to manual gearboxes have simply been referred to as gearbox oil. That distinction tended to recognise that the liquid in the auto box did more than just lubricate; it was also engaged in the crucial role of providing torque coupling enabling the transfer of the drive. In contrast, there has tended be a casual assumption that the role of the liquid in the manual box was very much that of a lubricant… and nothing more????
So it is interesting to note that, in more recent times, there has been a growing trend towards the use of the term “fluid” as applied to manual boxes, mostly in the form of the acronym “MTF”– manual transmission fluid, which sits neatly alongside the well-established term “ATF” – automatic transmission fluid. Of course, we in the UK need to remember that in North America, the term “gearbox” is one which is generally less-used in automotive terminology and the term “transmission” is preferred.
But even in the UK, owner handbooks are now increasingly telling owners that their manual gearbox needs a specific MTF fluid, for example, Honda’s ‘MTF3’, rather than simply a grade of oil based upon an SAE viscosity specification like ‘75w-90’. This raises questions as to why this change as come about and just what is ‘MTF3’ and the like?
Well in the latter case, don’t expect to find out much about MTF3 from Honda; it seems Honda have made no technical information available about their product. So let’s move on to Texaco’s MTF94, and in this case, rather more information can be found. MTF94 was originally formulated by Texaco for specific use in some of Rover’s transmissions, and presently, MTF94 is still specified for some Range Rovers and a detailed specification for this fluid can be found here http://www.kewengineering.co.uk/Auto_oils/ROVER%20MTF.pdf
Moving on to the issue of why we are seeing the use of transmission fluids rather than our friendly old gearbox oils, a good website to visit in this context is that of the American company Red Line Oil . http://www.redlineoil.com/Default.aspx
Red Line produce two transmission fluids designated MT-90 and MTL and these have proved a popular choice with many car owners in the US. The Red Line website offers a number of technical White Papers; http://www.redlineoil.com/content/files/tech/MTL%20and%20MT-90%20Tech%20Info.pdf http://www.redlineoil.com/content/files/tech/Gear%20Oil%20Tech%20Info.pdf
related to transmission fluids and these are well worth reading, highlighting as they do, the role of the fluid in the smooth operation of the synchromesh. The point that can be highlighted here is that just as automatic transmissions use the fluid both as lubricant and torque transfer agent, in manual boxes, the fluid lubricates and also plays a key role on the synchro action.
Not being transmission engineers, it is not fully clear to us quite what has changed with gearbox design and construction, but it does seem to be the case that the correct choice of fluid for modern transmissions is more critical than was probably the case in the past. When you see comments like “some oils are too slippery for the synchromesh”, you know that you’re getting in specialist territory!
And returning to the oft-asked question about whether or not MTF3 and MTF94 are “the same”, what we can say is that are own measurements have shown that MTF94 is a slightly heavier (more viscous) fluid. It might be the case that most of the time, many transmissions would operate satisfactorily with either, but when the temperature drops much below 5°C, some will feel distinctly smoother with MFT3!
| Viscosity/cS | |||||
| SAE | API | 40°C | 100°C | VI | |
| Valvoline 75W/90 | 75W/90 FS | GL5 | 100 | 14.5 | 150 |
| Redline MT90 | 75W/90 FS | GL4 | 90 | 15.6 | 185 |
| Castrol driveline | 75W/90 SS | GL3-5 | 85 | 15.2 | 189 |
| Castrol SMS | 75W/80 FS | GL1-4 | 67 | 11.5 | |
| Redline MTL | 75W/80FS | GL4 | 56 | 10.6 | 183 |
| Texaco/Rover MTF94 | 75W/80FS | GL4 | 53 | 10.5 | 191 |
| Amsoil ATF | 39 | 8 | |||
| Castrol ATF | 37 | 8 | |||
| Honda MTF3 | ? | ? | 30 | ? | ? |
Note; The Honda MTF3 data was obtained at Edotek; all other values are taken from the manufacturer’s data.
It’s interesting to compare the specs of Red Line’s MTL with MTF94.