1)- Just use the modern car/light truck oil & change often (nothing wrong with that).. ANY of the modern emission compliant oils will work well as long as they meet the OE spec & are changed often..
2)-Use a modern diesel speced oil like 15w40 or 15W50 (usually a 15W oil is a diesel speced oil.. The oil’s rated for HD or diesel use still have a much better anti wear package due the higher bearing & cam loads on the diesel engines & don’t have conventional catalytic converter to worry about. In fact Harley recommends a diesel rated oil for replacement..
3)- Use a motorcycle or air cooled rated oil as those have a littlie higher resistance to high operating temperatures & better acid handling additives in them ( on all the tests I have seen, the motorcycle only oils aren’t that much better to justify the additional cost & in some cases aren’t even as good in certain cases. Just because the MoCo or oil company recommends it doesn’t always mean it is better, it just means the MoCo (or oil company) will make money on it & it will meet the minimum requirements of the engine design…
4)-Use a quality synthetic oil, preferably a long drain type (like Mobil 1).. Those synthetics use a much more precise base stock so have less polymers & viscosity improvers in them so are a lot better at shear resistance,, handle high operating temps better,, (usually) have superior additive packages in them, & do flow better at low ambient starting temperatures… The only down side I can see to using full synthetics in a wet clutch motorcycle engine is the possibility of the clutch slipping in extreme use conditions.. So far the feed back on this & other M/C sites seems to indicate this isn’t a big issue (at least for normal light duty usage)..
JDC