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Engine Oil vs Gear Oil

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Old 07-16-2006, 04:02 PM
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Default Engine Oil vs Gear Oil

We've all heard the usual terms about the "weight" of the oils used in our bikes. 20w-50, 75w-90, 10w-40 - all these terms are bandied about with ease for most of us. But what do they really mean?

Without getting totally scientific, the terms represent viscosity measurements. They aren't "weights", as we like to call 'em. Viscosity is defined as: "the property of resistance to flow in a fluid".

We say that lower "weights" are thinner, meaning they flow more easily, or have less resistance to flowing. We want both the ability to easily flow during start up, and more resistance to flow at higher temperatures. The multi-grade oils we use give us both these attributes, and help our engines live much longer than in the day of using straight 50w, 60w, and even 70w in an old Pan or Shovel motor. But there are some trade-offs with the multi-grade oils. They "shear" more easily than straight oils. Shear is when the oil's actual molecular chains are sliced into pieces. When that happens the viscosity drops, and the oil provides less protection.

That's just a bit of a basic oil primer. In the past year or so HD, as well as several oil manufacturers, have started recommending the use of their 20w-50 synthetic engine oil in more than just the engine. You can use it in the primary to lube the chain, and in the transmission, where it must operate in a gear-grindin' environment. So, how can HD say to use 20w-50 stuff where they once called for 75w-90 stuff? Wouldn't it be too thin to do the job?

Here's the answer: Viscosity is measured by different scales for engine oil and gear oil. The two systems aren't directly comparable, number-for-number. Here's a reasonably accurate comparison between the two systems. Please remember that I said "reasonably accurate", as there is some overlap.

SAE Engine Oil SAE Gear Oil

10w thru mid range of 20w = 75w

Mid 20w thru mid 30w = 80w

Mid 30w thru low 40w = 85w

Low 40w thru 50w = 90w

60w up = 140w

Looking at these ranges we notice that 50w engine oil is the same "real" viscosity as 90w gear oil. So it stands to reason that they might be substituted for each other. But remember that there are other factors involved than just viscosity. The ability to withstand shear, and additives designed for a particular end use are also important.

The shear aspect and the additive packages are why some don't think it's the best idea to use the 20w-50 engine oil in a transmission. There are reports of tranny damage where the 20w-50 was used. However, there are currently no scientific studies to prove the damage was the result of the use of those oils. You must make the decision as to your own bike.

Personally, I'll continue to use gear oil in my tranny. There are enough differences in the engine vs. tranny applications that I feel better going that route.

Add: Seems HD no longer recommends the Syn3 in trannies.

Last edited by pococj; 12-13-2008 at 12:34 PM. Reason: Get rid of the crazy "junk" inserted during the forum change.
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Old 04-01-2007, 12:25 AM
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Old 05-24-2007, 07:09 AM
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Default RE: Engine Oil vs Gear Oil

I did some thinking on what type of oil to use in my new '07 Super Glide and I deciced to use Harley's Syn3 in all three holes. I like the fact of just using one oil like I do in my '04 Suzuki Hayabusa. IMO if one oil can do it, why not use it? The Japanesse bikes have been using one oil for the engine, clutch, and transmission for decades with no problems. These bikes (japanesse motorcycles) also have wet clutches where the clutch wears andconstantly gets into the oil system, and this creates no problem.I have personally seen these high performance bikeengines broken down and the engines and transmissions are sqeakey clean with no damage.

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Old 05-25-2007, 08:31 PM
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Default RE: Engine Oil vs Gear Oil

Urban Cowboy, your choice may be fine BUT the Hyabusa, if I am not mistaken, delivers oil to the trans and primary UNDER PRESSURE whereas Harleys are a "bath" type system. It makes a big difference in pressure resistant requirements and , like the rear or manual tranny in a car, I think real gear oil is best. Why use a compromise lube when target specific products are available and, as far as I can tell, superior for that narrow use. Convenience? Do you carry primary and trans. lube with you? How do you check your primary level on the road? Doesn't wash for me, maybe convenient for the dealer? Even the MOCO in 06 and up bikes no longer recommends the syn three but, rather , their new trans/primary lube. Must be a reason since it is cheaper than Syn 3, the overpriced semi synthetic.

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Old 10-28-2007, 11:52 AM
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Default RE: Engine Oil vs Gear Oil

The question I have stated before is? Would you put gear oil in your engine? I did not think so. The rest of the thinking is up to you.
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Old 11-08-2007, 11:04 AM
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Default RE: Engine Oil vs Gear Oil

I was firm believer in gear oil in transmissions always used the stuff on my 02 dyna/ with some Moly added.Now In my 07 Flhx, I use the 20w50 Amsoil.....because the HD manual recommends GL-1 spec oil in the tranny. I was reluctent at first but Its been 5000 miles and my tranny not only shifts smoother,therewere nometal shavings on the drain plug when I removed it.I just go with oil spec the manual recommends.
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Old 11-08-2007, 02:08 PM
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Default RE: Engine Oil vs Gear Oil

I have used amsoil 75/90 in my 07 fxst and shifter better and ran quieter than when I first bought it, I tried the amsoil 20/50 but right away I could tell a difference, louder harder shifting, more noise in 5th gear and it seemed to get hotter. Someone suggested spectro 6 speed transmission oil so I tried it and that's what I'm gonna stick with. It's a toss up between amsoil 20/50 and mobil 1 20/50 in the engine and primary, they both seem to perform nicely. as for the syn 3, I tried it once in a pinch but the engine made more noise ran hotter and didn't seem to even run the same. I will definately stick with a quality true synthetic oil rather than the blend that makes up syn 3,
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Old 11-08-2007, 05:15 PM
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Default RE: Engine Oil vs Gear Oil

First of all any oil other than Harley is just asking for trouble because i dont care who the manufacturer is nobody on this planet has done as much testing on air cooled engines as Harley i have used H-D oils in both of my bikes since i first bought mine in 1978 and my Shovelhead now has over 200,000 miles on it with a 90' motor and the bike runs better than most. My newer bike is a 2005 FLHTC with a 103' motor and all i use is the regular 20w50 and the new lube for trans and primary and this bike has over 28,000 miles on it in less than 2 yrs the bike makes 115hp and runs like spot butted ape. So use H-D and you will never have a problem. If you put that gear oil in a bike that still has warranty it will void it because it's not a factory recommened oil.
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Old 11-09-2007, 10:19 AM
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Default RE: Engine Oil vs Gear Oil

I have been using Amsoil 20X50 in my 07 it has 21k on it and I haven't had any problems, BUT the gear oil makes sense. I think I'll try the Amsoil 75X90 this spring.. It may be a little much this winter when I'm riding in 30 degree weather.
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Old 11-09-2007, 10:48 AM
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Default RE: Engine Oil vs Gear Oil


If you put that gear oil in a bike that still has warranty it will void it because it's not a factory recommened oil.
Again (as in your other posts saying the same thing) please provide documentation from HD where they say this.
Here's H-D's position on the subject, published in the Fall '07 Enthusiast magazine and posted by another forum member:

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