The high should be set for max rpm, then opened up until it starts to slow down a little...
Some brands of oil make more black than others, the engines I run on my test stand have left little black spots on the door behind the stand..I run all different brands of oil, Klotz, Amsoil, Pennzoil, Dolmar, Poulan, whatever is handy...I always put more in than it should have...I LIKE extra oil....
The engine will idle at 15 or 1600 on the ground with a 17 or 18 inch prop, but when coming down from higher speed it probably loads up...Try not idling all the way down until the plane is on the runway....Look at the thread on the MVVS 160, lots of info on synthetic oil there...