When you’re just changing the oil it’s never a problem, as the pump stays primed.
However, after my CB750 engine rebuild I had a nightmare getting oil pumping around the engine and up into the gallery with the hex plug at the end. I tried filling that with oil, and had built the oil pump immersed in oil, but it was not circulating, and I was very fearful of killing my engine - someone on the board had bought a freshly rebuilt bike with a ruined engine because it had been started without oil circulating. The previous owner had given up in despair after that, he wasn’t doing it again.
The solution, given me on this board as to remove the oil pump again, and pack it with Vaseline. That worked a treat, much to my relief. The Vaseline then dissolves into the oil without a problem.
I think Trigger may have some experience in this field.