Because that's what it's made for. Gear oil and engine oil are rated different. The numbers don't represent the same things. And gear oil has different additives made for your tranny.
And guys are wondering why their trannies are going on them. There is even a thread about it.

 

Amsoil.

 

Its what I use and I could not be any happier... I feel 150% protected and its well worth the cost to me
** correction: I personally use the 20W50HD PowerPack, also check racer warehouse website for best pricing I've found with free shipping
http://www.racerpartswholesale.com/p...yclePowerpacks

 

K i did it says never shift down from 6 to 5th above 50mph.... But on the tranny oil it says to use Formula + (a dino oil with a weight of 10W-40 I think) or Syn 3 20W-50

 

I’m not trying to start a war…. In fact I love learning new things…But regardless of the numbers, there is a big difference in 85W-140 gear lube and 20W-50 Engine oil. I work in the industrial field and we use ISO ratings as appose to SAE weights. 90 weight gear oil and 50 weight engine oil (because they are measured differently) are both ISO grade 220. Just for reference the 140 is ISO rate 460. When I worked at a Ford dealer as a line tech I had to rebuild many of the NP transfer cases that were designed to run on ATF because people would replace the ATF with gear oil. That is a different subject because they had a bullshit plastic pump in them that would strip with the thicker oil. All the metric bikes use engine oil in their gearboxes. The strange thing about this whole conversation is I find myself (regardless of my owners manual) feeling like gear oil is the better fit in the tranny. Truth be told I would switch if I knew I wouldn’t screw something up.
Flame Away

 

Steve.
I can tell you that putting shockproof heavy in my trans eliminated the clunk when going into gear,made neutral much easier to find. It whined a bit when I got it with 5000 kms but shockproof heavy eliminated the whine. All in all I feel it's the best thing for our trannys.
I learned about it here in this forum. Many members are using it in their 6 speeds. Try it. If you don't like it drain it. Simple.
And for those of you using motor oil in your primary it is imperative that it's a Xw-40. Doesn't matter if it's a ow-40,5w-40,10w-40,15w-40 etc. These oils generally do not contain friction modifiers. Thinner oils do contain moly and may ruin your clutch. I use rotella t 15w-40. No friction modifiers and high in anti wear agents and detergents.
And it's cheap so drain it often

 

Whatever works for you and keeps you happy. Im amsoil fan, 20w50, gear oil.

 

I have found over the many bikes(HD's) and the miles and modifications I have done : REDLINE has given me the best results as compared to the Wear and Tear upon inspection.

I use regular 20/50 Redline in the engine (not V- twin, less $$ and does great)

MTL in the primary, and is wet clutch approved

Shockproof heavy GEAR OIL in the tranny

********************************************

Other brands of Lube : My Opinion here

If Meets or Beats the specks of HD engine oil, it will work

For the clutch and primary, IF it says wet clutch approved it will work

GEAR OIL is for a tranny and that should be what is used there. Your choice on brands, but Please!!!

Not Engine Oil

signed....REDHEAD

 

Another vote for Amsoil 20w50!

 

I prefer Mazola