TC,
I agree with part of your comments - I've also owned quite a few non-Harley bikes over the years, and they had unit construction (same cases and oil supply for engine,primary drive, and trans). The difference between them and a Harley is that they were designed from day one to use a common oil supply, with materials, clearances, etc. appropriate for that system. Harley's, at least Big Twins, were designed as separate units with separate oil supplies, and the MoCo required different lubes for the engine and the trans until just recently.
A common misconception is that gear oils are "thicker" than motor oils. This comes from the two different rating systems in place for the two products. If you compare the actual kinematic viscosity in centistokes @ 100°C of a 50 wt motor oil and a 90 wt gear oil, you will find that they are very close. The real difference is the type of additives. Gear oils tend to have more extreme pressure additives and higher shear resistance, among other things.
Jerry
Some examples:
Mobil 1 V-Twin 20w50 Motor Oil - 17.7 centistokes @ 100°C (212°F)
Mobil 1 75w90 Gear Lube - 15.2 centistokes @ 100°C
Mobil 1 75w140 Gear Lube - 25.3 centistokes @ 100°C
Jerry, I stand corrected, sir....I did at least admit that my logic was suspect

I appreciate your post. Out of personal curiousity, what, if anything, changed in the HD tranny's from the point at which they were recommending gear oil, to the point at which they began to recommend syn3 in all holes? I ask this in all seriousness, as this is my first HD in many, many moons...so many moons, in fact, that the motor I was running was a panhead. This is why I made contact with the friend at Mobil (no, I'm not getting free oil, guys

), and asked him a million questions about what to use, where to use it, etc...I am fanatical about keeping my cars, trucks, outboards, etc maintained, and believe a few extra bucks to use the best possible product is well worth the investment, so do a lot of research/question asking about this kind of stuff. Coming from a background of growing up around my granddaddy's gas station, working on my own old cars out of necessity, and 18 + years in the heavy commercial HVAC industry (1K and up ton chillers, etc), I always enjoy hearing what someone in the know has to say. Thanks!! [smiley=xyxthumbs.gif]