My -10 has the standard Van's recommended two blade Hartzell metal prop. I have noticed that for a particular fuel flow, I get about 4 kt faster TAS at a low RPM (2150 vs 2500). The engine seems happy enough at the low RPM but I wonder if there are any RPM limitations for this engine/prop combo. Is it safe/sane to run it even slower? Anyone with thoughts on this subject?