I'm certain that there are motorcycle brand specific forums that go into extreme detail on many facets of the brand including lubrication needs.
Got to a KLR650 site for instance and their lubrication recommendations are much like the machine itself. Economical, effective and reliable. The seasoned KLR650 riders - serious Iron Butt kind of guys who do all of their own work and thoroughly understand their machines recommend HDEO lubricants more often than anything else. No fancy designer oils for those guys.
The Valkyrie site has a mix of opinions for oils. Some are for the synthetics but most recommend HDEO. Despite the Valkyrie's sophisticated engine, it's such a well balanced and under stressed piece that it's fairly easy on oil. Honda recommend 8000 mile OCI with conventional oil. I have no reservation whatsoever running Shell Rotella, Delo 400 or Delvac in it. When you factor in that the transmission is also sharing the same oil, you would think that it would be harder on oil than a Harley engine.
As you mentioned, Harley engines shouldn't have the need for specialized oil. Any oil with sufficient viscosity to handle air cooled engine temps will do. Rather than spending big bucks for designer oils, I'd be running HDEO in those things with 2500 mile OCIs.
From my exposure to the HD guys I know, they have very little mechanical understanding of the machine. None do their own work, not even oil or spark plug changes. When you have no understanding of the machine it's easy to default to the " If I spend more money on oil, I'll get more protection." syndrome. Not necessarily true.
You can bet that there are HD owners out there that know the machine inside and out, do all of their own work and know exactly what the bike needs to run well without breaking the bank. The sad thing is that those seasoned guys are not the same people that the HD guys I know go to for info.
My friend with the HD brings his bike to a well respected HD mechanic. My buddy is going through the dreaded oil selection dilemma. I told him that I run Rotella 15W40 in just about everything. The mechanic also advised him to use HDEO. Buddy goes to the donut shop for a get together with other HD owners who have the same mechanical skills as him and they advise him otherwise. So he goes back to the mechanic and brings a few quarts of Amsoil with him.
I just don't get it. The mechanic knows more, many times over, than all of them put together but he's over-ruled by guys who've never touched their bikes with a wrench????
What's the thought process there?