I'm running synchromesh right now in my t56, so far very good results. Prior to the change, I was looking at tranny/gear fluid viscosities.
Factory fill for me was mobil-1 atf, viscosity of that is 35/7.6 cSt at 40/100C. I too put a short throw shifter (hurst) in and found it harder to shift, but there was a problem getting into 2nd and downshifting from 6th-5th under easy driving conditions with the stock shifter. And my car is a 2002 SS, less than 10k miles. So I still need to throw the stock shifter on to give the synchromesh a fair evaluation.
I got a pdf tech paper from quaker state website on their synchromesh- says meets gm part# 12345349 (spec# 9985648 this is "GM synchromesh") or GM part# 12345577.
Havoline, quaker state/pennzoil are all making synchromesh fluid now and I'm sure it's all the same stuff just different labeling.
Viscosity is 41.6/9.08 cSt at 40/100C; SUS is 209.4/56.7 at 100/210F. So the synchromesh is in the 30wt motor oil range.
Some more numbers:
Redline D4atf is 33.5/7.4 cSt, and amsoil atf is similar I believe. redline hi-temp atf is 53.9/10.0cSt; MTL is 50.8/10.2cSt and MT90 is 89.2/14.7cSt. redline 75w90ns which redline recommends for some gearboxes (maybe yours?) is 92/14.8cSt.
Those who use synchromesh in the T56 6speed report good results and I think it's mostly due to the higher viscosity. And those who use MTL also report good results, again synchromesh & mtl similar viscosities- basically a little higher than the atf's.
I don't know anything about honda gearboxes, especially those that say to use motor oil. I wonder what effect motor oil would have in my t56 (a synchromesh 6-speed gearbox)? In any event, being your car is old it's very possible the synchro is bad and no fluid will fix the problem but I would say give the synchromesh fluid a try if it looks like it meets your gearbox specs. hope this helps.
PRODUCT DESCRIPTION
QUAKER STATE® SYNCHROMESH FLUID is a synchromesh transmission fluid designed for certain manual transaxles and manual transmissions used by General Motors or Chrysler. QUAKER STATE® SYNCHROMESH FLUID is formulated with high quality paraffinic base stocks, a fluidity modifier, multifunctional performance additives, corrosion inhibitors, a foam suppressor and a shear stable viscosity index improver additive. It provides excellent oxidation stability, low temperature performance, excellent synchronizer performance and compatibility with yellow metals, such as bronze, brass and copper components found in manual transaxles and transmissions. This product will satisfactorily lubricate General Motors or Chrysler manual transaxles and transmissions from -40°C to +150°C.
APPLICATION
QUAKER STATE® SYNCHROMESH FLUID is specifically formulated for synchromesh transmissions used by General Motors requiring General Motors Part No. 12345349 (Specification No. 9985648) Synchromesh Transmission Fluid or Chrysler transmissions requiring Part No. 4874464 (Specification MS-9224). It is also recommended for use in General Motor transmissions requiring General Motors Part No. 12345577. It is listed in the Pennzoil Lubrication Recommendation And Capacities Guide as “GLS.” QUAKER STATE® SYNCHROMESH FLUID is NOT intended for all GLS applications. You must verify the manufacturer’s part number, indicated by a superscript number and found at the end of the vehicle application listing.
BENEFITS
• Exhibits excellent low temperature performance
• Meets GM Specification 9985648
• Suitable for use in GM manual transaxles and transmissions requiring GM part number 12345349 or 12345577.
• Meets Chrysler specification MS-9224
• Suitable for use in Chrylser transaxles and transmissions requiring part no. 4874464
• Excellent synchronizer performance
• Excellent yellow metal compatibility
