Assuming like most you are concerned with gathering the crop, I would advise starting tight and fast on the cylinder and loose on the chaffer.  This will get the crop in the grain tank.  Work on cleaning up the sample once you get it in the field.  Air is going to be challenging too...oats are about the only crop I can think of off hand where you actually have enough air availible to be "too much." 

Kill stalling the machine in the field is the best way to figure out what is going on.  Stall it, then look in the walkers.  If you have clean, intact heads you are set just right.  If there is still seed in the heads, need to be more agressive.  If the heads are ground to dust, need to back off, most likely on speed.  I would expect the concave to remain in fully closed postion.