You definitely want 20% oil content in a YS or longevity will suffer.

Now about the sound you heard, was it like the sound of a piece of tissue over a comb or a kazoo? I ask this because I had an issue with an engine years ago that exhibited that sound and would die at throttle down intermittently. It was a piece of grass that had made its way in onto the throttle barrel on the side that leads from the crankcase to the intake valve. If you fly off of grass it might be worth pulling the throttle barrel for a look see.