One of the most commonly brought up questions from our customers is what gear oil to use in their freshly built Honda Transmission. Our reply to this question has always been: DON'T! "He must be crazy", you are probably thinking, but there is a simple explanation for this answer:
After a comprehensive analysis, we can conclude that a large number of drivers out there use the completely wrong fluid. The lubrication system in the Honda transmission was designed to use engine oil, and this has not been changed for any of their transmissions. What this means is that, in order for the gears, shafts, bearings, hubs/sliders to be lubricated as intended, a low viscosity transmission fluid must be used, regardless of whether it is synchronous engagement or dog engagement.
Basically, you MUST use the correct transmission fluid in a Honda Transmission and should not use a performance gear oil as it is too thick and does more damage than good. Performance Gear Oils also lack the additives necessary for the smooth running of the synchronous engagement system, as well as a lot of them containining chemicals which disintegrate the brass synchro rings. This is not common knowledge because customers are led to believe marketing and/or technical advice related to your typically large hypoid transmissions, NOT your specialist Honda Transmission.
For most cases where the vehicle is an unmodified Street Car, the OEM Honda MTF (Manual Transmission Fluid) is more than adequate. The problem arises when you start putting either higher power or higher rpm shifting through the transmission. Not only must you still keep within the tolerances of the oem lubrication system, you must also protect the transmission from shock loads. Using Performance Gear Oil is the Number 2 Reason (after incorrect installation) that is directly responsible for most transmission failures (i.e the Cause), subsequently leading to a part failure (i.e the Symptom).
Unfortunately, most consumers tend to follow the advice of your so-called "Performance Specialist" who are only concerned about the symptoms, rather than the cause, and usually end up recommending big-brand (and often expensive) performance gear oils. This is a 100% NO GO on a Honda Transmission. Although GL4 & GL5 Rated Gear Oils contain shock protection additives, they are also too thick for the oem Honda lubrication system. The engagement system (synchronous vs dog) is irrelevant in this case. Lubrication is the key word here.
A basic analogy would be: Oil is your transmissions blood supply and needs to flow smoothly. Clog it up with cholesterol (i.e thick gear oil), and it will have a heart attack. Not enough Iron (i.e shock additives), and it will get anaemic/weak.