Hey Bluesfan, Gear lube viscosity vs motor oil
Harley Oil & LubricationDiscuss Hey Bluesfan, Gear lube viscosity vs motor oil in the Harley Tech & Harley How-to forums; I read, heard several times that gear lube viscosity was rated different than regular motor oil. That 75/90 gear and 20w50 are about the same weight. And that is why ...
I read, heard several times that gear lube viscosity was rated different than regular motor oil. That 75/90 gear and 20w50 are about the same weight. And that is why HD can run the same syn oil in all there cavities.
And in all honesty gear lube seem a lot thinner than it used to be.
I have tried to NOT spread this as fact but have never found anyone or anyplace to answerer this for me.
Personally I think they are graded on the same scale, but don’t really know.
Do you have any input?
Different, and completely arbitrary, scales with no scientific or engineering basis whatsoever.
Viscosity measurements I use most often is Kinematic, it is stated in Centistokes at a temp, typically 40C. This is a repeatable, science/engineering based number.
SAE crankcase oil / Kinematic(approx)
SAE Gear Oil
----------------------------------------------
M 10W 22-32
G 75W 20-45
-----------------------------------------------
M 20W 32-70
G 80W 45-80
--------------------------------------------
M 30W 70-110
G 85W 80-125
--------------------------------------------
M 40W 110-150
G 90W 145-340
----------------------------------------------
M50W 150-250
G 140W 340-800
===================================
The lines mnean nothing, just helping you track across.
These were scaled off of a card on my desk, There are more accurate values out there, but you can see what is really going on.
Side note: A fellow I know is on the SAE committee that makes this stuff up.
With the advent of 0W-20 oils (real viscosity 10-70 depending on temp) they have a problem.
How do you go thinner than zero?
The potential answer, scrap the system.
According to him, the 30W system was picked out of the air long ago. Before that you used heavy and thin or some such. I had a 51 Ford as a teen,(my brothers car, but he was hurt so it became my daily driver)
Stuck to the inside of the glove bosx were instruction on oil and kerosene ratios to use at certain temps. I want to remember that it used winter oil and summer oil instead of weights, but my memory is subject to revision on that.....
Wow, that is interesting, I have ask many people this question and never got a real answer.
I know about the thinning the oil, we used to do the same thing in reverse back before power steering on steering gear boxes that leaked.
We used to pump in grease to thicken up the gear lube when I worked at the City Service station (pre-Citco).
I have printed this out, and will keep it in my desk.
Now I just need to figure out how I can work this into my conversation next time I talk to my real smart gearhead friends;-)
Quote:
Originally Posted by BluesFan
Different, and completely arbitrary, scales with no scientific or engineering basis whatsoever.
Viscosity measurements I use most often is Kinematic, it is stated in Centistokes at a temp, typically 40C. This is a repeatable, science/engineering based number.
SAE crankcase oil / Kinematic(approx)
SAE Gear Oil
----------------------------------------------
M 10W 22-32
G 75W 20-45
-----------------------------------------------
M 20W 32-70
G 80W 45-80
--------------------------------------------
M 30W 70-110
G 85W 80-125
--------------------------------------------
M 40W 110-150
G 90W 145-340
----------------------------------------------
M50W 150-250
G 140W 340-800
===================================
The lines mnean nothing, just helping you track across.
These were scaled off of a card on my desk, There are more accurate values out there, but you can see what is really going on.
Side note: A fellow I know is on the SAE committee that makes this stuff up.
With the advent of 0W-20 oils (real viscosity 10-70 depending on temp) they have a problem.
How do you go thinner than zero?
The potential answer, scrap the system.
According to him, the 30W system was picked out of the air long ago. Before that you used heavy and thin or some such. I had a 51 Ford as a teen,(my brothers car, but he was hurt so it became my daily driver)
Stuck to the inside of the glove bosx were instruction on oil and kerosene ratios to use at certain temps. I want to remember that it used winter oil and summer oil instead of weights, but my memory is subject to revision on that.....
I will defer to Bluesfan but I have always thought there was a huge difference in chemistry between motor oil and gear oil. The viscosity is only part of the story. Wet sump bikes have been using motor oil in the trans forever.
Really would like to know more. The tech on this can get deep fast.
Blues, please correct me if I have been taught wrong.
The following I was taught years ago by a retired Navy SeaBees.
Gear oils have anti shear additives that are much "stringier or stickier" that motor oils.
They cling to the metal and lube gears like they were designed to.
Motor oils have much lighter, or thinner versions of those same additives in addition to additives that combat acid formation caused from piston ring blow-by and water vapor that naturally condenses inside the engine.
They flow easily and protect engines like they were designed to.
After 3k or so miles the anti-acid additives are used up and gone in a typical motor oil and the oil becomes acidic and literaly acid etches the parts inside the engine.
Gear boxes usually do not have to worry about piston blow-by or water vapor so they can go 6k miles or even more before the additives are ground up and pulverised by the gnashing of the gear teeth.
Once a multigrade oil is hot, it's at it's higher viscosity anyway.
TBone, 80W140 is too heavy. It'd be more like 20W something outrageous.
75W90 is closer to 20W50.
That is indeed hgow we get away with using 20W50 in all 3 holes.
Keep reading.
Wrighttuurn, you are correct, there is a large difference in the chenmistry between gear oils and motor oils.
That is the biggest reason I advocate any good 20W50 Syn for the motor (we meet one of the conditions that make it worthwhile, high heat or extended drain intervals) whereas for the primary and tranny I like a good fossil made for the purpose. Like BelRay tranny lube and primary lube. There is just no way that a motor oil can have the EP (extreme pressure)c additives that a gear lube has. Ever wonder why gear oil has a unique odor? That's right, additives that motor oil cannot use.
dls8, you are on the right track. Except that the oil companies have gotten better at their job. So the old 3,000 thing is long gone.
Example, my wife has a Volvo S60R. It's a hot rod with a hi pressure turbo. With semi syn oil the recommended change is 7,500 miles. That alone says a lot about how oils have changed for the better.
Other Euro cars are similar, some have oil change intervals that make me look twice.
Whereas we in Amaerica are wedded to our Jiffy Lube recommneded 3,000 mile changes. Total overkill for an average car.
Note that HD has increased their miles too.
And, Name brand oils are blendeded with better addiditves these days. Fighting grades from the discount aisle in 7-11 are a crap shoot long term but will get you home.
BluesFan, great stuff.
Pretty sure it was in one of Donny Peterson old oil thread debate article, he stated that the worst thing for a HD transmission was to heavy a gear lube. That is kept the inner bushing on the output shaft from getting any lubrication. Wore it out, then the seal was not far behind.
Quote:
Originally Posted by BluesFan
TBone, 80W140 is too heavy. It'd be more like 20W something outrageous.
75W90 is closer to 20W50.
That is indeed hgow we get away with using 20W50 in all 3 holes.
Hey Bluesfan, Gear lube viscosity vs motor oil 
I learn something every day. 
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