I've seen this discussion come up several times and thought I would just post this to show graphically how the grading systems for oils and gears are different...and similar.
Gear lubes and motor oils are graded with different numbers only to try and avoid confustion between the two. It makes them more recognizable, but their viscosities actually are very similar and do overlap.

Here is a viscosity chart I got of of BITOG. Chart

It is a little dated as it does not have the 2 new gear lube viscosity grades that came out in 06, the 110 and the 190. What they did is take the 90 and the 140 and cut them in half to create the 110 and 190. Here are the new viscosity requirements for the 4 grades stated in kinetic viscosity at 100*C, the ASTM D445 which is stated on all oils and gear lubes in cS(centistokes)
90 - 13.5-<18.5cS
110- 18.5-<24cS
140 - 24-<32.5cS
190 - 32.5-41cS

As you look at these on the graph you can see that a 110 gear lube is almost the same range as a 50 weight oil.
An example is our 20W-50 oil. It's KV at 100*C is 20.1cS.
The severe gear 75W-90 is 16.7cS
The severe gear 75W-110 is 20.4cS
the severe gear 75W-140 is 25.7
We do have a straight grade 190 with a KV of 37.6. These aren't recommended for use our bikes but just wanted to show the KV value.

Each oil on the market stating a specific grade has to have a KV for the ASTM D-445 test fall in these ranges, but the exact numbers can vary. So you could have a 90 weight with a KV of 13.6, but it does have far to fall to shear out of range, so finding lubricant that falls in the middle or upper level of its range is beneficial in that if it does start to shear, hopefully it will not fall out of range and compromise protection.
There are shear tests in both of these studies if interested in looking at them.
Motorcycle oil study
Gear lube study

Hope you find this helpful.
Heath

 

Heath, I'd like to see a UOA on SVG 75w110 in a Harley 6 speed after 20,000, 25,000 and 30,000 miles. That would be meaningful to a lot of people on here who think synthetic gear lubricants are expensive.

I run redline shockproof heavy in mine and change it every 2 years and am thinking of going 3 years and doing a uoa to prove you can buy a gear lube for $10 that will go several years which makes it much cheaper and less aggravation than formula +

 

I agree. It's amazing how clean the tranny is when drained. I venture to stay it will hold up just fine. If I ever come across one, I'll definitely post. I personally run the 20W-50 oil in mine and will test it if I ever get that many miles I'll start at 10k and go from there.

 

The SVG 75w110 will go 20,000 easily in your trans. Try a gear oil. You will never go back ot a motor oil in the trans. Much smoother & quieter.

 

Even though I might be an idiot, I'll continue to follow the maintenance schedule I've used for the past 40 years....

Yeah I know, "new fluids are so much better", "you're stupid to change it that often", "what a waste of money", but I don't care....

I like to tinker with my bikes and it give me a nice warm fuzzy feeling when I'm running new fluids.

I change the engine oil primary fluid at about 3,000 miles, and the tranny fluid at around the 9,000 mark, or every 3rd oil change.

But I also change my tires (both front and rear) at around the 10,000 mark, even if they aren't bald...

I was curious and checked the Owners Manual for my 2003 Softail to see what Harley recommended for fluid changes...

Using their suggested fluids they recommend changing the oil in all 3 holes (engine, primary, and tranny) every 5,000 miles.

 

Been a sticky up in the top for awhile: https://www.hdforums.com/forum/oil-a...-gear-oil.html

 

My apologies...pay me to look a little next time

 

No apology necessary; your chart is easier to read than what I could put up way back when.

 

I just changed my engine oil to amsoil syb. Now I am changing my trans to amsoil 75/90 or mobil 1 75/90, if I can find here.

 

Autozone carries Mobil1 75w90. AAP carries it too.