Valvoline 4-stroke motorcycle oil. Available EVERYWHERE: Autozone, Advanced Auto, Walmart, etc. It's usually on a separate shelf from all the regular oils. It's designed for Harleys, which are flat tappet engines, just like ours. Been running it for 5 years now.
On another note, I use 10w40 instead of 20w50. Lowered the cylinder head temperature by 20 degrees with that single change. Exact same oil, just lighter weight. Oil temp is unchanged, as it is thermostatically controlled.