The most commonly used oil is Rotella 15W-40. It is inexpensive, readily available in 1-gallon jugs, and is JASO compliant. Almost everyone uses it.
I used to use it. @pdwestman
and I have been running lab reports on our oils. I had some sad times with my engine and, after rebuilding the top end, changed over to the oil Paul uses. That's the Kawasaki 10W-40. Paul's lab reports were always better than anyone else's and so far my results are better than they were when my engine was healthy and running on Rotella. Still not as good as Paul's but improving. Paul's engine is, um, worn-in.
It is more expensive than the Rotella, but not horribly so. Rotella is usually on sale at one of the stores masquerading as an auto parts store, or at Walmart, for around $12-$13/gallon. The Kawasaki oil, last I bought it, was $23/gallon from Rocky Mountain ATV. Sounds a lot more expensive, but some motorcycle oils run $4-$10/qt.
Really, the only two important things are that it be JASO compliant and any oil is better than no oil. JASO compliant simply means that it has the things that are considered by the Japanese motorcycle industry to be important and doesn't have the things that are considered detrimental. That spec covers all the way up to 170hp/10K rpm Hayabusas. Any oil is better than no oil means that, in a pinch, I'd put anything I could find in it rather than run it low. The lowly KLR with its neck-snapping 35hp and 7500 rpm redline would probably run forever on 30wt Raylube...