Just curious how you know "Honda MTF does not like heat and will shear down very quickly..."?
This feels very spammy to me, but assuming you're not a bot, this is a good opportunity to review the NSX trans and fluid to help others on the forum for future reference.
The Honda MTF II (White Cap) is an ATF-derived fluid that uses an additive package to make it suitable for synchronous manual transmissions. Because it is ATF based, it is quite thin and because it uses a conventional base stock, it is susceptible to heat-related shear. The fluid will shear down rapidly to an even thinner, water-like consistency if the trans oil temp goes above around 250F. Like any conventional oil, once sheared, the fluid loses much of its lubrication properties. These conditions are only encountered on the racing circuit- no amount of street driving can get the NSX trans that hot. Thus, for NSXs that are not tracked, I continue to recommend the MTF II because it is fantastic at lubricating and cleaning the transmission internals and provides excellent shift quality. Owners should stick to a 24-month change interval. However, if you track your NSX (especially in hot weather), I strongly suggest changing the MTF as soon as possible after the track day to preserve the life of your synchros, bearings and gears.
All early NSXs left Tochigi filled with a simple, conventional 30W motor oil. That's right, just plain old dino oil. You can still use regular motor oil in your transmission with no issue. As Honda developed newer transmissions in the 90's, they created the MTF I to work with those designs. It was basically the same 30W oil with an additive package that improved function in the newer transmissions. Honda back-certified it to work the with the NSX, so Acura dealers started filling the MTF I instead of motor oil during the transmission fluid service.
Then, in the early 2000's, Honda was in the early stages of its "Earth Dreams" program. One outcome of this corporate effort was to push to improve the fuel economy of its fleet. At the same time, Honda customers were reporting notchy shifting in cold weather with the MTF I. I personally experienced this with my Accord 6-speed. As a result of these pressures, Honda created the MTF II. They moved to an ATF base that was a thin sub-20W oil (like all ATF). The thinner viscosity solved the cold weather shifting (it flowed much better and faster) and the MT cars picked up some MPG due to the lower fluid pressure from the thinner oil. This oil also was back-certified for the NSX and remains the OEM dealer fill to this day, at least in the USA. Honda created a MTF III for Europe, which goes back to the heavier 30W oil, but I'm not sure why. All Honda MTF fluids are made by Idemitsu.
To get an idea of what the NSX transmission requires from the fluid, let's look at my 5-speed. Unlike many MTs of the era, the NSX MT (both 5-speed and 6-speed) uses an internal oil pump. It's basically a mini version of the larger pump inside the engine. Here's my oil pump during teardown.
The pump pressurizes the transmission fluid from the sump and passes it through a wire-mesh filter. This filter is designed to capture only large debris- basically anything large enough to interfere with the internal components. Here's my old filter- you can see lots of small bits of metal dust, which is normal.
The pressurized fluid is then sent to the countershaft and mainshaft, where it enters the shaft and squirts out of small holes to lubricate the free-spinning gears/bearings. The NSX does not use external oil squirters like the GR6 transmission in the GT-R, for example. It oils from inside the shaft.
On the countershaft, the oil enters from the bottom and makes its way up. Here's the oil control port on my 5-speed:
And here's the mainshaft port, which runs from the top down through the shaft. Interestingly, it's plastic.
Once the fluid makes it to the shafts, it squirts out of tiny holes. You can see them on my NSX-R countershaft below
Here, we have the mainshaft 3-4 synchro blocking ring, with another oil squirter hole visible.
Thus, the NSX MT requires pressurized oil to flow through tiny ports in sufficient volume to adequately lubricate the spinning shafts. But, that's not the only method. While the gears likely get some oil from this process, they are primarily lubricated by being semi-submerged in the trans fluid sump. The countershaft sits lower in the fluid than the mainshaft, but both are roughly halfway dipped in the fluid bath. Here's a crude diagram explaining what I mean:
There are two important lessons from this photo. First, the fluid must also be able to "cling" to the gears as the teeth emerge from the oil bath during their rotation. It cannot froth or foam up. Second, look at the oil pump- it's driven by the final drive ring gear. This means that if your NSX is sitting still and idling,
the trans oil is not circulating! The car must be moving for the oil pump to work. Thus, when you first start the car, you're relying on whatever is still clinging to the gears and shafts. As a result, especially if your NSX has been sitting for a long time, I strongly recommend driving slowly in 1st gear for at least 30 seconds before shifting so that the oil pump can get oil on everything.
Finally, here's my differential. The NSX uses a wet-clutch style LSD that compresses many clutch plates together to engage and disengage the differential locking feature. It shares the same fluid as the transmission. The transmission fluid must be able to provide the correct friction coefficient to support this process and not cause the plates to slip too much...or not enough.
Notice the one thing that is
not on the transmission? An oil cooler. The NSX transmission is convection cooled by the fins on the bottom of the housing. That's it.
So what does this all mean for transmission fluid? Essentially, the NSX requires a transmission fluid that:
- Is thin enough to flow through several tiny ports in sufficient volume to adequately lubricate the spinning shafts; and
- Is thick enough to "cling" to the gears as the teeth emerge from the oil bath during their rotation without foaming; and
- Provides the correct friction coefficient for proper LSD locking; and
- Lubricates all of the parts with an acceptable rate of metal wear; and
- Does not corrode or damage brass synchro rings.
That's a lot to ask from a oil and I find it amusing that a plain old 30W dino oil does all of these things just fine. So does Honda MTF II, except when it gets hot. How do I know? See below.
In discussions with one of the Honda engineers who designed the car, he advised that the MTF II should be changed after each track session due to its inability to cope with heat and that this is commonly done in Japan. Testing this theory, I tracked my 91 for a day in 96F ambient heat, drained and sent the MTF II to Blackstone for a UOA. It came back that the oil had sheared down to about a 5W viscosity- basically water- and the notes said something like "Do not drive your car with the fluid in this condition" and that it was something they commonly see with the MTF II after a hard track session. Since then, I always changed my MTF after a track day. That was over 10 years ago.
That experience led me down the long rabbit hole of finding a MTF that can do the 5 things listed above for the NSX and not need to be changed after an HPDE session. The folks at Synchrotech recommended the Torco MTF for the NSX transmission because it is a little thicker (~8.5 cS) than the MTF II (~7.5 cS) and the add package helps resist shear much better. They still recommended changing it after every 5 track days or so. That was also many years ago.
Since then, Synchrotech has released
their own blend of MTF for Hondas, which is made by Torco and is much more shear stable versus the vanilla Torco MTF. It's specifically formulated for their carbon-synchros, but is also fine to use with the OEM brass ones. They continue to recommend vanilla Torco for stock Honda transmissions that are street/occasional track cars. For race cars, they recommend Torco RTF, which is a full-synth 30W (~10.5 cS) GL-6 fluid and will last an entire season in the transmission. MFactory also uses Torco and Synchrotech MTF in their Honda transmissions and race cars. Neither shop recommends the Honda MTF II for anything other than daily driving because it shears so badly (and quickly) under heat.
Finally, after discussing with the folks at Comptech many years ago, they advised that the thermocouples on the transmission coolers for the World Challenge NSX cars routinely reported trans oil temps of 300F and that heat in the NSX transmission was a real issue (hence the coolers). Although I didn't drive my 91 close to a World Challenge car level, that info appeared to jive with my UOA and the advice from Japan.
Also, here's an amusing segment from the NSX VTEC Club of Best Motoring, where Tatsuru Ichishima from Spoon alludes to the same issue. He admonishes Tsuchiya for not changing his NSX-R's "mission oil" more frequently and points out that the NSX has no transmission cooler.
So yeah, don't use Honda MTF-II if you plan on driving on the track. Or, if you do, change it as soon as possible after the session. Other fluids that will work for the NSX manual transmission are:
Torco MTF
Torco RTF
GM Synchromesh "Cocktail": 2 qt Synchromesh + 1 qt Friction Modified
Synchrotech MTF
I'm going to be doing a UOA (Torco MTF) on my 500-mile break in soon and will post the results here. I expect to see a lot of copper and iron, since the synchros and gears are new.
