I purchased 10W-40 Castrol Power 1. I would have as easily chosen Mobil Racing 4T, Mobil's MA/MA2-rated oil, but they only had two quarts in stock. They only had 10W-40 in the the Power 1 but I'll order some 5W-40 for the next oil change. 10W-40 should cover the temperature ranges I expect to operate in (its rated for use in 14˚F to 104˚F) but since I'll be using the CF500 for ice fishing, I'll go with the 5W-40s for its lower temperature range, just in case.
The CF Moto branded 5W-40 oil in the oil change kits they sell is rated JASO MA2, by the way.
Back in my motorcycle road racing days (mid-1980s WERA C and D Production, D Superbike), I used BelRay lubricants. They were one of the first companies to promote four-cycle motorcycle-specific motor oils: BelRay EXP, if I remember correctly. I remember mail-ordering it to put it in my new 1979 Triumph Bonneville.