20W-50, especially in the single cylinder and during the hot summers.  Thicker oils better.  Also I use honda oil (honda, suzuki, and yamaha all relatively the same oil).

Wally world here in PA has MC oil.   I got Mobil 1 MC oil there.  Best price in town for it.  Look where they sell the helmets.

Rotella T  15-40 walmart, about $9.00 a gallon. Cheap enough to burn some & not cry, good enough to keep the engine alive.

Any major brand 10W-40. As long as it doesn't have the "energy conserving" logo. I am just a little leery of store brands, such as WalMart's "Super Tech." Otherwise, Pennzoil, Castrol, Quaker State, Havoline, Valvoline, or whatever. I weigh price heavily.

Many years ago I was reading a magazine, which had a column called "Ask Smokey". Smokey was Smokey Yunick, of racing fame. The question was essentially "What happens if I mix oils?" His answer, parapharased: "The Army operates the biggest fleet of motor vehicles in the world. The Army buys a lot of oil, and every oil supplier wants to sell to the Army. The Army requires that any oil it buys conform to MILSPEC. One of the requirements of MILSPEC is that any oil must mix with any other oil, in any proportion, without ill effects. The obvious reason is so the motor pool can just pour in whatever oil they have without worrying about what oil might already be in the vehicle."  He went on to add that if you happened to mix viscosities, you got something that sort of split the difference. That answer made so much sense that I ceased worrying about the brand of oil I bought.

I use Mobil 1  20w-50 V-Twin Motorcycle Oil.  Hot here in the south.  Wish I could find it cheaper than $10 quart.