May I join your discussion?
Several of the posts above have noted that HMC's MTF seems similar to motor oil. I agree. And for decades, that's precisely what HMC recommended for its manual-transmission cars, motor oil -- specifically, 10W-30 weight motor oil. In this regard, several of my current Honda owners' manuals state: "If Honda MTF is not available, you may use an SAE 10W-30 or 10W-40 viscosity motor oil with the API Certification seal that says 'FOR GASOLINE ENGINES' as a temporary replacement." The "temporary replacement" caveat is because (quoting the manuals) "[M]otor oil does not contain the proper additives, and continued use may cause stiffer shifting. Replace as soon as convenient." (See, e.g., the 2009/2010 Accord Owner's Manual.)(Emphasis added.)
The owner's manual language fascinated me. Could one really use straight motor oil? And when HMC reformulated its MTF approximately three years ago, I wondered what HMC had added to its old formula, if anything, to address owner's complaints about poor shifting and gears grinding. It appeared (from VOAs on this site) that HMC has simply added a huge dose of calcium to its old formula. And, as others have noted, HMC's MTF is still pretty thin stuff.
I decided to try a few experiments. Since other boards had reported decent results using Amsoil's MTF (a 5W-30 weight concoction), I decided to try it. And it worked pretty well, improving the shifting "feel" noticeably in two Civic Si's, particularly when temperatures were near freezing or below. This caused me to wonder whether synthetic 5W-30 or 10W-30 motor oil might work just as well over time. Further experimentation was called for.
In light of the caveat in the owners' manuals concerning the potential for stiffer shifting over time, I decided that instead of straight (synthetic) motor oil, I'd mix a quart of HMC's MTF (part no.08798-9031, white cap) with a quart of Mobil 1 10W-30.
I can report that after approximately one year and 6K miles in an '02 Honda Si with 26K miles on the odometer, this nutty concoction seems not just adequate or "good" but great. It works so well that I actually considered starting a business to sell the stuff. (Turns out the cost of the bottles (!) would make a "boutique oil" too expensive to sell in the limited numbers I could envision.) But the shifting feel is excellent, and equally important, the improvement is consistent over time, distance and temperature. The long-term consequences remain to be determined, of course, but the difference between Honda's straight MTF and this blend is like night and day.
