Syn 3, mobil 1 v twin, and amsoil 20/50 wt oils have a Gl-1 rating which seems to be what Harley says their trans needs to be happy. And given the different types of lubes people seem to use in them, it appears you could run straight 50 wt, 20/50 wt, 75/90 wt, 75/250 wt oils and they won't notice the difference. While the Harley trans is about as close to bullet proof as it can get, the biggest issue I ever hear about them are seal leaks. I (personally) think it's a bit thin for that application.
I'm thinking this is in the 20 to 30 weight range? Some use it in the primary chaincase as well. Note that some use B&M Trickshift in their transmissions. Don't run this in the others because they don't shift as slick with this combination. ALTHOUGH - I do have a Dyna that I've run Mobil Delvac (dino) 15W-40 in the transmission and in the primary chaincase for the last five years. On the big twins I like Rotella or Delvac dino 15W-40 in the primary chaincases and have been using Mobil 1 75W-90 LS in the transmission because this is the combination (for me) that gives me the smoothest shifting. Used to use Castrol but last several years have been using the Valvoline VR1 Racing 20W-50.
We've been running a 20W-50 in Sportster shared primary chaincase/tranmissions for years. If you're the kind of guy that changes oil in his Harley 3k OCI's and the transmission and primary 5K OCI's then yes, you could use a dino 20W-50 in all three holes. It's probably way overkill unless you like extended drain intervals. Amsoil recommends their 75W-110 gear lube for Harley big twin transmissions. Harley says that you can go 20,000 miles on the transmission fluid change if you use their Syn3. Not unusual for a Gold Wing to go 200,000+ miles. Many of the newer Japanese bikes spec 10W-30. They seem to run many trouble-free miles on 10W-40. Think about this: All of the Japanese bikes have a shared sump with the crankcase, transmission, and clutch basket. You would probably want a multi-viscosity 50 weight. Would a 50 weight motor oil work just as well? Yes, and no.
Harley evo transmission oil change manual#
Also - take a look at automotive manual transmission lubes and ATFs and they're in the 20 weight range and many folks never change them. Not exactly high-tech fluid - but it is representative of many automotive manual transmission lubes (just thicker). There's a virgin oil analysis of the Formula+ out here somewhere and it's something in the 40-50 weight range, with virtually no additives. Yes, 75W-90 gear oil is roughly equivalent to a 20W-50 motor oil. Gear oil is graded on a different scale than motor oil. You're not the only one that has these questions. Wouldn't a straight weight 50 accomplish the same thing that a thinner gear oil is going to do? Is that why they recommend a 20/50 syn, but not a 20/50 dino? Understanding that the transmission is going to create shear issues with oil. If you look at the specs on mobil 1 75w-90 and Valvolines Syn 75w-90 you find that they are no where near a 50 weight on the viscosity chart. And there is plenty of people saying that a 75w-90 is thicker than a 50 weight motor oil. Somewhere along the line I got the idea that a 90 weight gear oil is roughly the same viscosity as a 50 weight motor oil. So Harley says use Formula +, or a synthetic 20/50 in the transmission.
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