All you need in the bikes is a GL-1, which the oils qualify for as well. The GL-5 gear lubes work, it just has more extreme pressure additives that aren't needed but wont' hurt anything.
Some gear lubes also contain friction modifiers. These are what you usually don't find in motorcycle oils as they can cause problems with clutch slippage. The preferred lube for this combo primary/tranny would be a oil/lube without friction modifiers but has you have heard from some who do use gear lube, so far they aren't having any trouble.

 

For those interested, here is a breakdown of the various API "GL" ratings...

Up to individual owners, but I prefer a good GL-5 rated oil for the transmissions on all my Harleys and the manual transmissions on my other 4-wheel vehicles as well.

API GL-1, oils for light conditions. They consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils are designed for spiral-bevel, worm gears and manual transmissions without synchronizers in trucks and farming machines.

API GL-2, oils for moderate conditions. They contain antiwear additives and are designed for worm gears. Recommended for proper lubrication of tractor and farming machine transmissions.

API GL-3, oils for moderate conditions. Contain up to 2.7% antiwear additives. Designed for lubricating bevel and other gears of truck transmissions. Not recommended for hypoid gears.

API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have small displacement of axes, the gearboxes of trucks, and axle units. Recommended for non-synchronized gearboxes of US trucks, tractors and buses and for main and other gears of all vehicles. These oils are basic for synchronized gearboxes, especially in Europe.

API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials if they correspond to the requirements of specification MIL-L-2105D or ZF TE-ML-05. In this case the designation of class will be another, for example API GL-5+ or API GL-5 LS.

API GL-6, oils for very heavy conditions (high speeds of sliding and significant shock loadings). They contain up to 10% high performance antiscuffing additives. They are designed for hypoid gears with significant displacement of axes. Class API GL-6 is not applied any more as it is considered that class API GL-5 well enough meets the most severe requirements.

 

Okay so I've done a lot of work on my bike, but I seem to be confused with the primary & tranny oil. I recently did an oil change (primary) and have been told that it is 'shared' with the tranny, so I assumed that I had actually changed both... is this true? Or do I need to open up the derby cover and change out the oil in there, using the same stuff I used for the primary? (by the way I used 20w50 AMSOIL- it is by far the best. Had a friend with a digital thermometer and his bike was 15 degrees cooler with AMSOIL than with the MOBIL-1!!!).

Thanks,
RoNiN (photo coming soon)

 

The primary & tranny are the same hole. It takes 1 quart.

While a quality synthetic oil will typically run 15-20* cooler than a fossil based oil, Mobil-1 & spAmsoil will produce them same temperature results when tested under the same conditions. Your "friend" is blowing smoke up your a$$.

 

?? Now I'm more confused. The normal oil change I did (with taking off the plug, putting a new chrome oil filter on, etc.) isn't that the primary?? and then the tranny oil is the derby thing?

And the engine was 15 degrees cooler, not the actual oil. He actually switched it back to Mobil cuz he didn't believe it, but after testing and switching again it ran the engine at 15 cooler!

 

No, what you changed is the engine oil. The primary/tranny are a separate single item.

If you're going to work on your own bike, you need to get a Service Manual. It's a lot cheaper than what it will cost you if you screw up something as simple as an oil change.

 

Awesome, thanks for your reply! I'm still learning all the differences between my bike and the cars I work on... looks like I'll be changing the primary tomorrow! I found a great guide on how to do it that- I have posted a link here for anyone else who would be interested. It's been 5000 miles since this has been done!

http://www.thesportsterblog.com/?p=1704

 

+1 for the Mobil-1 75w-90 Gear Oil. Mobil 1 is good stuff, and engine oil for the engine, gear oil for the tranny sounds like good reasoning to me.

 

BelRay still makes the proper fluid for Sportsters and it's called Sport Fluid. I've always use Sport Fluid or 20-50W.