oil sending unit

I am trying to locate the oil sending unit on a 90 trans am formula with a 305.

Do you mean Oil pressure sending Unit or Oil Pump?

not sure which one it is i'm just fixing it for a friend and would like to know where both is. My dad says its on the back onf the blo0ck but where. Its know to be a common problem with the 305's.

OK since this is what I think your talking about cause the oil pump is never really called a sending unit, Here is a image of the oil pressure send unit and its located on the driver side of the vehicle above the oil filter: http://files.automotiveforums.com/gallery/watermark.php?file=/503/144075oil_pressure.jpg .
If it was the oil pump you where looking for its in the oil pan to the rear of the engine it looks like a round disc with a metal pipe neck sticking out. If your friends problem is that he is registering 0 oil pressure when the car is started then it is the pressure sending unit. If the problem is there is a surge of pressure on start up and then drops to like 5 PSI then it is a spun bearing or worn oil pump.
John