If you are running 20% oil I don't think you have anything to worry about.  On a four stroke the oil is not very diluted so a little goes a long way.  As long as the bearings are getting a good coating I wouldn't worry.  I would check to see if the cam and rockers are getting good lube.  Also I might see if some petrol oil or castor oil will mix with the fuel and your synthetic as synthetic glow oils will not cling to the steel bearings as long as petrol or castor and that could cause rust.

When you are getting better performance from gas than glow it often means you were not using the best glow plug temp.  Often with four strokes the plug is too hot because of the worries that it will cool too much during the intake stroke.  However with 5% you may have needed a hotter plug or smaller prop.  The RPM indicates that possibly the prop was too small as most 4 strokes deliver their best power above 9000, some over 10,000, but I am not sure of the best RPM for this engine.  Also the engine would deliver the best power at the same RPM regardless of the fuel, I would adjust that by switching to a smaller prop.

Also since you have at least some oil that comes with the 15% gas I would actually lower the oil percentage, maybe 17% or so.  This would also increase performance a tad.  With all gas I would go with 12%, unless it has roller rod ends, then I would probably go with 37:1.