I won't recommend a brand, any brand will work fine. Synthetic, non synthetic, car oil, motorcycle specific oil, whatever. The Rebel engine dates back to the late '70s and is not picky about oil. I do recommend 20w50, unless you live in a really cold place. It has better load bearing ability than thinner oil, and shifting seems to be a bit easier. I also recommend changing it every 1000 miles, which means synthetic can just be a waste of money. The Rebel has a small air cooled engine with no oil filter and is hard on oil. I use conventional 20w50 and change it every 1000 miles and it runs just fine. While I don't recommend overfilling it, I do recommend filling it completely full, right up to the top mark on the dipstick. The Rebel does not hold much oil to begin with, and if it burns a little, or you have a minor oil leak, you could run out of oil and destroy your engine.
2016 Rebel, 2013 Royal Enfield B5, 2009 Genuine Stella, 2006 H-D Sportster 1200, 2006 Vespa GT200, 2001 Yamaha XT225
"I refuse to believe in a risk free society where the thrill of living is traded for the safety of existence." — Nick Ienatsch