By Eric Kangas
As a "gearhead" (registered P.E. in Mechanical Engineering) oils have been an interest of mine for 20+ years. I have have been certified with the Society of Tribologists and Lubricating Engineers. However, I still learn every day.
For lubrication the number one factor is viscosity. My most vivid experience is with the 5 speed transmission in my Nissan truck. Nissan specs 75W-90 GL-4 gear oil. One time I used 80W-90. The transmission felt like the synchros were going bad. I had to double clutch to downshift. I put 75W-90 back in and the transmission shifts like new again.
There are tolerance ranges on viscosity specs. Comparing gear oil viscosity to engine oil viscosity is roughly:
75W-90 gear oil would be roughly equivalent to a 10W-40 engine oil
The first number with the W designates viscosity at low temp, the second number designates viscosity at high temp. Oil viscosity decreases (thins) significantly at high temperatures. As such viscosity modifiers are added to increase viscosity at high temps. It is a balance of an oil that will flow at low temp (when highly viscous) to provide adequate lubrication and an oil that will provide enough viscosity for effective lubrication at high temp (when the oil thins or loses viscosity).
To give you a rough idea, a 10W-30 oil is about 2.5 times as viscous at 100F as it is a 212F.
The GL-5 designation for gear oils is questionable for use in some transmissions. However, most gear oils on the shelf today, except for motorcycle specific, have the GL-5 designation.
Theory aside, here is my experience with transmission oils in my GasGas:
Mobil 1 15W-50 (light clutch drag, clutch growl under heavy slip)
Spectro Gear saver 10W-30 (some squealing clutch engagement noise, clutch growl under heavy slip)
Spectro 80W (light clutch drag, some clutch growl under heavy slip)
Valvoline ATF FA (low clutch drag – clutch growl under heavy slip)
Valvoline gear oil 75W- 90 (limited use – worked very well, stopped using it since the additives reportedly may not be to friendly on transmission bearings)
Silkolene 75W (noticeable clutch drag, clutch growl under heavy slip)
Castrol MTX (no issues noted with clutch drag or clutch growl under heavy slip - MY 2nd PLACE RECOMMENDATION)
AmsOil motorcycle specific synthetic 0W-40 oil. (no clutch slip, no clutch noise at all under heavy slip, very low drag - in every regard better than even Castrol MTX) MY TOP RECOMMENDATION.
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