what is the difference between 75w90& 75w140

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The difference is viscosity. When cold, they're about the same (that's the 75w part). When hot, both of them thin out but 75w-90 thins out more.
 

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 Originally Posted By: live_to_hunt
Trans gear oil for a Harley some guys say 75w140 some say 75w90.
Try Redline Shockproof Heavy which they claim is 75w250. I use it in my 6 speed FXD and it works just fine. Plus you can run it for 20,000 miles easy. The 30db comment is no joke. A heavier gear lube in a Harley trans will quiet them down nicely. If they made a 75w500 I would try it.
 
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Harley-Davidson recommends their Formula+ - which is roughly a 50 wt lube. 75W-90 gear oil is roughly eqivalent to 20W-50 motor oil. I see that Amsoil recommends their MCV 20W-50 as a replacement for the Harley Formula+ (or 20W-50 Syn3) But why is 75W-110 recommended over the 75W-90? Noting that Amsoil markets both of these grades. ??? HD Formula+ VOA
 
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In 2003, the viscosity scale was revised. They took the old 75w-90 range and split it in half; the thinner half retained the 75w-90 label, and the thicker half was re-labeled as 75w-110. If you take a look at the gear lube chart link that Pablo posted, you can see the old rating scale vs. the new (click the big blue boxes at the bottom of the chart). It's a great illustration. So basically, if you see a 75w-110 today, it would have been called a 75w-90 back then. Conversely, any oil that was on the thick side of the 75w-90 range back then would be called a 75w-110 today. Amsoil MCV is 20.1 cSt @ 100 degrees C, which makes it equivalent to a 75w-110. On the pre-2003 scale, though, it would have been called a 75w-90. Does that make sense?
 
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 Originally Posted By: kballowe
Harley-Davidson recommends their Formula+ - which is roughly a 50 wt lube. 75W-90 gear oil is roughly eqivalent to 20W-50 motor oil.
Not really. A 75w90 is more like a 15w40 motor oil. Formula+ is basically an 80w140 GL-3 gear oil.
 
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 Originally Posted By: Harley Anderson
So is there any advantage to running the SVG instead of MCV? I advised quite a few people to run MCV in all three holes on a twin cam like Amsoil says to do.
Well natch you can't use a gear oil in the engine, but pretty much always (100% of the time) a gear oil will protect a gear box or rather transmission better than an engine oil. Nothing wrong with MCV in the tranny, per se, but I think some of it is the convenience of using the same fluid in all three holes.
 
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 Originally Posted By: G-MAN
 Originally Posted By: kballowe
Harley-Davidson recommends their Formula+ - which is roughly a 50 wt lube. 75W-90 gear oil is roughly eqivalent to 20W-50 motor oil.
Not really. A 75w90 is more like a 15w40 motor oil. Formula+ is basically an 80w140 GL-3 gear oil.
Hmmmmm. The Mobil 1 75W-90 gear oil is a favorite of many for the Harley transmissions. Is this a bad idea? I change mine out every 10,000 miles or so but I don't have any high-mileage bikes. The highest so far is my Road King with this combination and 68,xxx miles. And just a quart of Rotella in the primary chaincase each 5,000. I have done this in many other bikes, but they usually are sold/traded before the 50k mark.
 
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