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I don't know about GM cars but I've never seen a rubber gasket on a drain plug.  Usually drain plugs want to be torqued on with some force and a rubber gasket would leak if torqued to that extent.  I have seen soft metal gaskets (aluminum) and rubber coated metal gaskets, but never rubber.

Some (many?) oil pan gaskets can be very expensive to replace as often cross members and engine mounts get in the way requiring the engine to be unbolted and lifted in the engine bay.  Access to oil pans can be rather difficult, but I don't know about the Grand Prix. 

Another thing that's often easy to fix (depending on access) are valve cover gaskets.  If the leak is coming out around the head then this could be the problem.  Also, check for spark plug wire boots covered in oil.

The first step to fixing an oil leak is finding an oil leak.  Clean the engine from under the car as best as possible (try a degreaser to get the nasty stuff off), drive it around, and inspect.  Ceretainly some photos of oil spots on the ground won't be much help in troubleshooting, except to verify that yes, the oil is indeed leaking out (as opposed to being burned in the engine or leaking in to the coolant).

gunny i dont know your car too well, but my car idles at several hundred RPM's I believe. I'd need to double check it. whenever the engine is running its going to be more than 0rpm.

possibly a faulty O2 sensor? or new air filter needed.

Hey Gunny, when was the fuel filter last changed? One other possibility might be water in the gas. Try running a can of "Heat" brand fuel additive on the next fill-up. O2 sensors have nothing to do with idle. They don't even work until heated up by the exhaust gases and are only used to fine tune the air/fuel mixture and then all they do is switch on and off repeatedly. They normally get replaced from being plugged up rather than by failure.

Strange idle typicaly caused by something in the fuel system, replace the fuel filter and see if that helps.  If the car is fuel injected then get a bottle of cleaner ($1-$5) and put in the gas tank.  Clogged injectors can cause a lot of problems.  Some vehicals are just strange when they idle.  My old 89 Bronco usualy idles anywhere from 800-1500 RPM and never stays consistant.  My mom has a 97 Mercury and its the same way with the not so smooth idle.  Also if your weather has been very cold like we've had down here then most vehicals are going to have a hard time idling till they start to warm up, till the past few days we were in the -5°F> range at night, both my Bronco and my F-150 required a foot on the gas for a minute or so before it would idle without sputtering or dying.

Re: bad idle, always start with the basics (the free stuff).

Open the air box and check the air filter.  Go over the entire intake and check for obvious vacuum leaks (look for vacuum tubes which may have come loose, re-tighten all the hose clamps, etc).  Pull all your plug wires and (if you have a spark plug socket and torque wrench) all your spark plugs and visually inspect them for anything weird.  With the plugs out turn the engine over once or twice and sniff for gas in the cylinders (do NOT leave the plugs anywhere near the holes or they'll ignite the gas vapors).  Check for spark.  Watch the idle controller and see if it's doing its thing while the engine is trying to idle.

Once you've done all that free stuff then try replacing things like fuel filters.

There is a chance (perhaps small) that the oil problem and bad idle are related.  If you've got a bad enough oil leak in the valve cover you could have plugs covered in oil which could be hindering your spark.  It's a long shot but it has happened.

I'll cross my fingers and hope for something as simple as a pan or valve-cover gasket. I think you can rule out head gasket or a cracked head, as oil in the coolant would also mean coolant in the oil (the oil would start looking like a chocolate milk shake).

My gut instinct, at 140,000 miles, says, "rear main seal".. 

hmmm rear main seal? the car should have 141,00 or 142,000 miles on it by now. The oil goes in Golden honey colored.... comes out looking the same. Nothing is leaking into the oil.... just oil leaking out. I'll just go out there tomorrow with my camera and take pictures of the various filters and oil leak etc...

Gunny