here's a viscosity chart from bob is the oil guy. a 75w90 equals to around a 12-15w40-50 in a motor oil.

 

the chart above is the main reason I decided to go with a gear oil rather than a engine oil. a gear is better for your trans than a engine oil any day.

 

On my Big Twin HD's.....the primary gets the 15w40, but the transmission gets 80w90 GL5 gear lube. Likewise, the engine gets 15w40 in the winter months and 20w50 in the warmer months.

But, that Sportster I had.....nothing but 15w40. It can't be that bad for it considering the sucess I had with it. On the plus side....15w40 was and still is inexpensive and can be found almost anywere.

BTW: Nice chart

 

Thanks

 

Nice chart, and it proves that 90 weight gear oil is definitely somewhat 'heavier' than 50 weight motor oil, even though not by much. 140 is obviously very much heavier.

The only thing about this chart that puzzles me for now, until I learn otherwise, is where it refers to gear oil as "SAE." If I'm remembering right, and I definitely could be very wrong though, gear oil doesn't have an SAE rating.

But again, I could be simply in bad memory about this and have it mixed up with something else; I don't know. I guess I'll have to go dig through my stuff out in my garage and see if I have any gear oil and it says "SAE" on it.

 

Most of the guys I know with EVO Sportsters (including a few HD certified mechanics) are running Mobil 1 75W-90 gear lube in the combo tranny/primaryoftheir personal bikes.

They could run the 'Harley Recommended Fluid', but they prefer to run the gear lube.

Some of them now have over 60,000 miles with zero related problems.

 

A good post from this forum on the subject of "oil weight" https://www.hdforums.com/m_601136/tm.htm

 

This thread is proof that Sportsters are girl bikes.

You guys argue like a bunch of chicks.

 

LoL.....

Now that was just cruel....

 

all oil post no matter if is a engine post or a transmission post, is a post that no one will ever agree on. if you see the facts , and people will give their opinions. so be it . use whatever makes you conferable. if you make a wrong decision on what you used, it was your decision. all I'm seeing is what about this or what about that. I made my decision and I'm going to live with it. I said my piece hope you all figure out what oil you want to use and and don't blame other for the decision you make.