Toyota Repair: 91 Previa after rebuild engine, oil pressure sending unit, low oil pressure

Question
After rebuilding an engine for a 91 Previa LE
engine started fine ran ok but the oil pressure
lihgt came on at 1000rpm or below the oil level was about right 6 quts in the oil pan if I rev up
the engine the light will go off, the first thing I did was changing the oil pressure sending unit
after I started the engine this time both oil level light and oil pressure light came on that pissed me off. I forgot to mention about the main bearings and rod bearings are in specs when I put them in. Now I took off the oil pump and inspect it for wear and tear and visually,it seems ok to me but I don't have the specs for gear to gear
clearance and gear to pump body clearance if you
do could you please send to me through my email.
I noticed that when I first stated the engine there's a noise of the motor running in the oil reservior area on the front of the vehicle it last about 1 minute, does it have to do any thing
with the lights that came on the dash? if the oil
pump is ok what else could it be?
The lights weren't on before I overhaul the engine and I used gear oil instead of motor oil because of high mileage (250,000+ miles)
Please help, thanks.  

Answer
There is obviously low oil pressure on start up, this could be caused by a worn oil pump, it is agood idea to replace the oil pump and the pickup when doing major engine repairs, did you check the oil pickup screen to make sure it is not partially clogged.
If not, replace the oil pump along with the pickup tube and then also replace the oil level sensor on the oil pan.