As said above... You will however need to keep the rpms up for good pump performance. 1/2 speed is barely a problem, but idling down on the already reduced pump speed will have it running at 1/3 or 1/4 speed. As the oil thins with warming, internal pump leakage [proportional to output pressure, but independent of rpm - and increasing as the pump wears] will start to overtake output. For example ... In a fully warmed system running at full pressure at 1/4 pump nominal speed it is likely you will only get 1/8 of nominal full rated rpm flow. Maybe less. At 1/2 pump speed youre pretty good - instead of the 50% capacity you will get around 40% at pressure. Not too bad.
Also, some types of pump are better than others for reduced speed service because they have less internal leakage. I have assumed it is a gear pump.
larry