Pontiac: oil leaks, valve cover gasket, oil pressure sending unit

Question
I and my wife own a 1990 Pontiac Grand Prix LE. It currently has 190,000 miles on it. I have noticed a very noticeable oil leak. The car is a front wheel drive model and the car is leaking fresh oil when it hits the ground. I believe it is coming from the oil pressure sending unit. Or could it possibly be valve cover gasket or oil pan gasket. I use about a quart a week. The dripping is on the firewall side of the car. Please help.

Answer
What I would do to first to try to identify the where the leak is coming from, is to simply inspect the engine.  I don't know how meticulous you have been about keeping your engine clean but with 190,000 miles on it I'm sure there some built up grease/oily stuff.  Try inspecting anyway on the firewall side of the engine since you say it is leaking on that side.  
If you can't see anything, then if you have a means to lift the car with a jack and some jackstands, slide underneath and check from there.  From underneath you will most likely be able to find the leak.  If you don't have jackstands to hold up the car you can put your van up with one side up on a sidewalk and the other side on the street, this should allow enough clearance to check underneath when laying on the street curb.

If you still can't find anything then you might want to pressure wash your engine and the underside of the engine (which will be more difficult.  Just make sure you cover the alternator, distributor cap if you car uses one and any other major electronic components.  Don't worry about connectors, they are waterproof.
Try and some degreaser at a local auto parts shop and spray after getting to the car wash place open up the hood and spray everywhere with the degreaser and follow the directions on how long to leave it.  Then just pressure wash everything as well as you can until everything looks VERY clean and without grease.  Try and pull the wand underneath the vehicle and try to get the lower part of the transmission/engine as well as you can.

Once you are done, drive home and if you leak is big enough to allow a quart loss per week then by the time your arrive home check all over again, and surely you will see the FRESH leak.

If you still cant find it and you really want to do it youself, they sell an additive that is not harmful to oil to add to your oil.  Run your engine for a while to get the stuff flowing and then turn it off.  This addative will show only using an ULTRA-VIOLET light, or Black light.  You will see the fresh oil GLOWING with this additive, and you will perfectly pinpoint the leak.  
I have never used it but my mechanic has used it on my cars to find leaks.

I have never had to do it myself, so I don't know if you can find it at an auto parts store, but you could ask for it.  

Some places to look though before doing all the washing and stuff is, the rim around the oil pan.  If any external oil connections are present, check that.  If you have a distributor, the base of the shaft sometimes begins leaking of the seal goes bad.  Could be also the valve covers.
Just make sure to take it slow and pay great attention to detail, that is the only way you will find it.

The oil pressure sending unit could be leaking since it is an external fitting so make sure to look for that.