1997 Cadillac Catera Oil Pressure Sender

282 miles ago and 30 hours of labor, my car had a big over haul performed by an ASE certified mechanic whom I personally know. Motor torn down and heads send out to be whatever it is that gets done to them, head gasket, manifold gasket, heater bypass valve, timing belt and some bearings all replaced as well as something else I can't recall at the moment. When all was back together, the oil pressure goes very low while at idle. So low that the light comes on. Would this problem be that the car also needs a new pressure sender and if so, where is that located so that maybe my husband can replace it vs. Taking it back to the mechanic. There is no GM repair manual by either Chilton or Haynes for this model! The car is a one owner vehicle and it's in fabulous overall condition inside and out; the only reason I spent the money to have this major repair done to start with and the fact the car is paid for. I'd like to get 4 more years out of it because the mileage I consider relatively low for the year; however, I realize that this model is German made based on an Opel design and it's known to have mechanical problems. What do you think? The oil pressure sender is a $31 part! Thanks so much.