RE: Too much oil.
Lille bror,
I do not question your test data, be assured of that. I may not have been too clear about the thrust differences, and how they come about. I will explain a bit more. I hope I am not being pedantic.
If air changes, so does engine performance, but typically, the propeller power demand changes almost equaly with the engine power output.
The propeller is an air brake, whose torque solely depends on air density and rpm.
So you may get same rpm, but different thrust figures (and hence power) as you have measured.
For tests like these, only rpm are of real importance because of the power/thrust.rpm correlation depicted above. When using prop rpm only on thesame test engine, most atmospheric variables are canceled out.