Transmissions that share the same oil as the transmission have a unique problem. Gear life would be extended if a hypoid type gear oil that had a true viscosity of at least 90wt. Clutch life would be at it's best if the viscosity of the transmission oil was about 1wt. The engineers from the major manufacturers have tested all types of oils and transmission fluids. They have found the types of oils and viscosities that give the best overall clutch and gear life without robbing a lot of power from the viscous oil drag created by the clutch basket churning heavy viscosity oils in the clutch compartment.
Put your engine on a dyno and fill the transmission to the recommended level with the recommended oil. Run the engine on the dyno and record the power and oil temperature. Now overfill the transmission by adding an extra pint. Run the engine on the dyno and record the power and oil temperature. Now overfill the transmission so that it is overfilled by 1 quart. Run the engine and record the power and oil temperature. Now drain the oil completely and fill with SAE 90 wt or 140 wt hypoid gear to the recommended level. Run the engine on the dyno and record the power. Now drain the transmission and fill with ATF or with one of the zero to 5 wt multigrade automotive engine oils. Run the engine on the dyno and record the power and oil temperature.
The summary of your test results will look something like this:
Best power.....had light viscosity oils like zero to 5wt or ATF filled to the recommended oil level
Lowest power......had the highest oil level of the oil recommended by the manufactures and or the 140 wt oil at the recommended oil level.
Lowest oil temperature ..........light viscosity oils like zero to 5wt or ATF at the recommended oil level
Highest oil temperature.........had the highest oil level of the oil recommended by the manufacturers. 2. Next highest oil temperature was the one that was overfilled by 1 pint or had the 140 wt oil and the recommended oil level.
Least clutch slip.........light viscosity oils like zero to 5wt or ATF filled to the recommended oil level
Had the most clutch slip..........140 wt hypoid gear oil and then the test that had the highest oil level of the oil recommended by the manufacturers
A clutch basket running in a high oil level can consume a lot of power. Power is consumed by the unnecessary turbulence and viscous drag created by the basket running in a higher than necessary oil level in the clutch cover. The power loss is converted to heating the transmission oil.
We cannot accurately evaluate which of the light viscosity oils would provide the longest gear life in our short term type of test in the lab. Accurate gear life type test require inspection of a large number of transmissions where the oil type, oil change intervals and severity of service can be well documented . Historically gear life has been extended with higher viscosity type oils or the newer synthetic type of oils where high oil temps are prevalent.
If we look at the above observed trends, I see trends that seems to match up with what most manufactures of two stroke engines with wet clutches recommend. The manufacturers typically recommend a multi-grade light wt oil like 10w30 or 20w40. The engineers have found that the light weight oils reduce clutch slip, minimize power life and give acceptable gear life.
I have used ATF type F in thousands of two stroke race engines over the years without observing accelerated gear ware. I would not recommend ATF in any of the Suzuki off road two stroke engines. I have seen what I consider accelerated gear ware in Suzuki's off road transmissions and I do not believe it is a lubrication related problem. I believe that Suzuki has been cutting corners the last 40 years or so on their gear materials and heat treat in their off road transmissions. I have not see the same metallurgical trends on their sport bike and other street bike transmissions.
For clarity in the above statements: "High oil levels were Oil levels above the manufacturer's recommended oil level.
