I have seen many post saying it's best to use 75-90 weight in the tranny and
almost as many post saying 75-140 is best. I phoned the folks at Mobile and
ask which they reccommended . Spoke with a gentleman who obviously didn't
think my question was serious enough to waste his time on - he fliped off an
answer that 75-90 was best for my '04 Heritage and that 75-140 was only for
heavy trucks. I emailed Spectro with the same question but they did not reply.
I don't know , maybe the question really is too stupid to bother with ??
Does anyone have advise on which is better ?
Thanks.

 

The gearbox of a Harley is not an overly hostile environment for any decent gear oil, so viscosity is of little concern. 75W-90, 80W-90, 75W-140, or even 75W-250 (like Redline Shockproof Heavy) will all work fine. Considering even the cheapest dino gear oil can go 50,000-100,000 miles in a truck differential without breaking down, any gear oil you put in your transmission will be more than adequate.
Wal Mart sells a quart of Rotella 75W-140 heavy-duty gear oil for less than $4, and this will provide excellent protection for the 1-2 year interval (and well beyond) that most people run their transmission oil for. Harley's factory fill Formula+ is also more than adequate; I had it in my bike for the first 1000 miles and it actually did more to silence the 5th gear whine inherent to the new 6-speeds than most of the other more expensive oils I've used in it, and it's only $6.95 a qt. I've used 3 different gear oils in mine (Formula+, Amsoil Severe Gear, and Shockproof Heavy) and couldn't tell a nickel's worth of difference between any of them for ease of shifting, driveability, finding neutral, etc.

IMO it's really not worth wasting a lot of thought on- use any good gear oil of the viscosity of your choice and you'll be set.

 

The only advantage to the 75-140 is you may get a little more noise abatement. I use the Redline Shockproof Heavy which is 75w250 and it quiets the gearbox about as much as possible. It also sticks to the gears better because of it's chemical composition. It is loaded with anti-wear additives and will last a minimum 20,000 miles. So you can go 2-3 years with 1 fill. The Harley Formula plus is the same viscosity as a 50w motor oil which is about equal to a 75w90 gear oil but shears down to a lower grade pretty quickly compared to a gear oil.

Me I would go with the 75w140 for the noise reduction. Many dealers have used Bel-Ray 85w140 gear saver oil for years and swear by it. But any of them will work just fine. As long as it says GL5 on the label it will protect your trans. The GL5 is the gear lube rating. Some people will tell you that you only need a GL1 rated 20w50 motor oil in the trans which is true. But it needs to stay GL1 for the entire life of it's use and GL1 oils in a gearbox will shear out of rating in no time. So stick with a GL5 because you won't shear it below your lubrication needs and it's the same price.

Ok, that's my $.02...

 

KISS Mobil 75/09

 

Many thanks for the advise.
Unk