ShyTorque
My handling notes (from 1997) say hydraulic, constant speed, propellor.
Slightly unusual operation, in that it is acceptable in some regimes of flight (e.g. the climb) to reduce rpm without reducing manifold pressure.
IO540
Assuming a standard CS prop however, this subject is massively over-cooked. You climb at max RPM to top of climb, then set RPM back to cruise RPM and leave it there until short final when it goes back to the "max" lever position.
Not on the DV20 - please see above.