A while ago I looked at the trickle of oil flow coming out of the feed pipe at the bottom of the pipe and was a bit nervous about starting it up. However, once I'd taken the pipe off completely and confirmed it wasn't blocked, I started it up. In a short time oil came squirting out of the return tube in the oil tank. Clearly for so much oil to return it must have got down through the pipe to the pump in the first place. I guess there must be some level of suction created by the pump, which is likely to be the way anti sumping valves work by pulling the ball off its seat once the engine/pump is turning.
As Goldie says, especially if the engine has been rebuilt, remove the PR valve and kick it without the plugs in until oil arrives at the PR valve, refit the valve and start the engine.
It's fine to be cautious, these things are precious.
Alan