I don't run K&N especially on the dual sport because of dusty conditions in which I ride. The street bike has a foam Uni in it due to price.
I simply run motorcycle oil from the local motorcycle shop, running whichever 10W-40 is least costly. If I wasn't close to a shop I'd do the same at another source. I may pay a bit more, but there is never any question about friction modifiers or anything else, besides I'm usually in the shop for the OEM oil filter too. Same could be done for synthetics too. Again no issues with the filter when it's OEM. I've seen where one company made a mistake in the O-ring on a canister that cost a guy an engine.
Actually as long as there are no additives that could affect anything, any oil can work in a bike, especially if you go with the ridiculously conservative 1950s change intervals most riders do. I run about 5000 miles and actually would go more, but if I use that generality I usually have the oil changed by at least 6000. I don't short hop my bike, it nearly always gets up to full operating temperatures, especially since I started using the bicycle for short hop stuff around town (better for the legs and wind). That cooks off any moisture and the oil will stay in good shape longer.
One thing I will say is that in 22 years in and around a shop I've never seen an oil failure. I've seen filters put in 250 Honda singles backward and aftermarket misfits, along with running oil too low, but never a true oil failure. So I'd never say Amsoil, Rotella, Castrol, or any other oils for cars won't work well. They probably will. I just play it absolutely safe, relying on the manufacturers and oil engineers to make the best call with their motorcycle product.
KLX650C, Zephyr 550
SR500, Bultaco Sherpa T