15w-50 synthetic in H-D Transmission?

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Thoughts about using a 15w-50 synthetic oil in a 2009 6 speed transmission? Kinda like a jungle with scattered info. Some say GL-5 rated oils (75w-90/140)are not neccessary and maybe too much. Amsoil is the only company I see that rates their 20w-50 m/c oil as GL-1. Mobile says the 20w-50 m/c M1 syn is better than their 75w-90 gear lube, as does Amsoil. Lastly, the viscosity chart would show a 15w-50 oil actually closer in spec to a 75w-90 gear oil than a 20w-50 oil. Reason is I can get a qt of M1 15w-50 for $5 from a buddy. Heck, I'm sure it would be just fine but just throwing out for opinions...
 
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Not sure if the JASO rating is going to be relevant as the oil is not going into the motor or the primary (clutch). I live in AZ where ambient air temps run up to 120 and even higher when running across the asphalt therefore I am running Redline Shockproof Heavy which is probably overkill. I would stick with gear oil myself as it appears more suitable for such an application than motor oil. Just my two cents worth. Have a good weekend.
 
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Shoot, I thought that was "heavy duty truck trans. I'm sorry, you are out of my area on motor cycles. Getting old is starting to get old lol Dusty
 
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Originally Posted By: troop
Strange thing, even Amsoil recommends their 20w-50 MCV motorcycle oil over their gear oil for use in the transmission. Why's that?
Never question the Amsoil Gods!! hide
 
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It looks like Mobil says its OK for the primary, but doesn't say to use it in a transmission. But then again there are metric bikes where someone is using it in a shares sump configuration.,,
 
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The first 19k on my sv650 were on 15w-50. I'm at 22k now (switched to schaeffers 9000 5w-50), still on the original clutch with no slipping.
 

troop

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True.. And most liquid cooled metric cruisers use 10w-40 oil in shared motor/transmission use. But they recommend 4-7k oil changes where as H-D says 20k on transmission. I'll use the 15w-50 syn in my motor and use some Amsoil 20w-50 in the tranny now that I have one quart of each on hand. Thanks ....
 
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Many folks are complaining about gear whine in the newer 6 speeds. Running a heavier lube quiets it down. AMSOIL recommends the 75w110 for the trans as an alternate. Likewise the 75w140 also works good. I used to use the 75w140 in the trans of my 99 BMW Twin because the shifting was so clunky. It did help a lot.
 
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You have to consider more than just viscosity comparisons between a gear oil and a motor oil. IMHO , drain that M1 and either use shockproof or golden spectro 6-speed oil.
 
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