There are many reasons why an engine might fail
markkal, loss of oil pressure is just one of them. In other failures, e.g. ignition, fuel malfunction, the prop would still be windmilling, and thus extracting energy from the air stream to drive the dead engine. In these cases, the propeller pitch is still controllable.
In the event of an engine failure caused by loss of all oil pressure, the engine would quickly sieze, leading to a stationary propeller, thus eliminating extraction of energy from the air stream, a lower drag condition than having the propeller windmilling at either high or low pitch. (This too, is very noticeable when you do it). Most CSUs allow the propeller to go to fine pitch following engine failure, some (the best) allow the propeller to go to the full feather position ..... the best of all.
An interesting scenario posed by
QJB, at first glance a fully open throttle butterfly seems to pose less drag, but would provide a much larger mass of air to be compressed within the cylinders ..... more energy extraction required! With the butterfly fully closed (closed throttle), most of the incoming air would stagnate in the intake, posing minimal drag. (This paragraph is my opinion, not a 'proven' situation, awaiting more informed opinion than my own).
Regards,
Old Smokey