Oil Pressure

I have a 1987 Nissan Pathfinder, 4X4, vg30 engine, automatic trans. The motors were swaped with another vg30 engine with 94000 miles. After driving it for 2 days the oil pressure drops to 10 after it warms up. It has a new oil pressure sending unit, and just had a new oil pump put in it at the shop and still doing the same thing, oil pressure dropping off. I was wondering what else could cause this, The head gasket was changed, heads milled, and new timing belt pror to the engine change.