Pontiac Repair: Oil Leak--, oil pressure guage, 1987 pontiac fiero

Question
I have a 1987 Pontiac Fiero--4 cylinder.  Ive noticed in the last couple of weeks I have a small oil leak--over the course of a 4-6 hour spell of sitting in my driveway or at work--a puddle about the 3-4 inches across has accummulated--I degreased the engine and from the looks of it, after driving it for a short time-the leak appears to be on the bottom side of the engine someplace.  My oil pressure guage is also reading a little on the high side---the car has roughly 100,000 original miles. Could any or all the gaskets--i.e.--valve cover, oil pan etc. be shot or might it be something more major--i.e---rear main seal or head gasket---if so--being a rear mounted engine--is it hard to change either of those--
                                 Thanx--

Answer
first thing to check is the oil pressure sending unit, also the valve gasket on your motor is another common source for leaks, and the oil filter is another possability. check those first a diagram is located here    http://us.f2.yahoofs.com/bc/412eb90f_c702/bc/My+Documents/42274471.gif?bfExvMBBQ...