So Harley says use Formula +, or a synthetic 20/50 in the transmission. 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. And there is plenty of people saying that a 75w-90 is thicker than a 50 weight motor oil.
That doesn't seem to be true. 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.
Understanding that the transmission is going to create shear issues with oil. Is that why they recommend a 20/50 syn, but not a 20/50 dino?
Wouldn't a straight weight 50 accomplish the same thing that a thinner gear oil is going to do?
You're not the only one that has these questions.
Gear oil is graded on a different scale than motor oil. Yes, 75W-90 gear oil is roughly equivalent to a 20W-50 motor oil.
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. Not exactly high-tech fluid - but it is representative of many automotive manual transmission lubes (just thicker). 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.
Would a 50 weight motor oil work just as well? Yes, and no. You would probably want a multi-viscosity 50 weight. e.g. 20W-50. Don't ask me how I know this.....
Think about this: All of the Japanese bikes have a shared sump with the crankcase, transmission, and clutch basket. They seem to run many trouble-free miles on 10W-40. Many of the newer Japanese bikes spec 10W-30. Not unusual for a Gold Wing to go 200,000+ miles.
Harley says that you can go 20,000 miles on the transmission fluid change if you use their Syn3. 10,000 on the primary chaincase. Amsoil recommends their 75W-110 gear lube for Harley big twin transmissions. It's probably way overkill unless you like extended drain intervals.
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.
We've been running a 20W-50 in Sportster shared primary chaincase/tranmissions for years. Used to use Castrol but last several years have been using the Valvoline VR1 Racing 20W-50.
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. 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. It shifts slicker than you-know-what.... Don't run this in the others because they don't shift as slick with this combination.
Note that some use B&M Trickshift in their transmissions. I'm thinking this is in the 20 to 30 weight range? Some use it in the primary chaincase as well. I (personally) think it's a bit thin for that application.
Random thoughts by Kevin. Your mileage can (and will) vary.