One time I did a head gasket job on a Subaru GL.  All the oil drained out of the galleries while the engine was out of the car.  When I put it back together, added oil, and tried to start, I got no oil pressure.  Eventually, after a lot of fiddling, I added an extra 4L of oil, cranked it for a few seconds, and as soon as I had oil pressure, drained the extra oil.

Have to be careful about how much oil you add is all - don't want to hydraulically lock anything!  You can do a lot of damage that way

tabe johnson/ex 87 westy