My 2000 GMC Sonoma P/U 4 Cyl 2.2L is leaking oil between the fire wall and the top back part of the engine.
I don't know exactly where, but that's where I can see the oil.
Please, can someone tell me how to repari this problem?
I have some knowledge of my vehicle so I pretty much understand where most of the parts are located on my vehicle.

In some minor cases, an oil treatment can help. In your circumstance, there is a good possibility that you have a bad head gasket. I would recommend that you replace the gasket and if you have never done it before, be sure to read up on it. Torqueing the screws back down in the correct specs is important.

Best thing to do is to try and locate where the oil is coming from before deciding how to fix the problem. An oil treatment can be nothing more than a bandaid in alot of cases. Replacing gaskets and seals is the best option.

edited....

ObseSSed and Canuck, Thanks so much for you replies. I really appreciate them.
I will all of your advises starting with cleaning the area and looking for where the oil is coming from.
Once again - Thanks.

I would think about making sure its not coming from the valve cover first. They leak often and the gasket is easy to replace and only costs about 5 bucks.

rear main seal. This is a pretty complicated procedure. Involves dropping the tranny out.

I would say rear main too.................

replace the seals.

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Is the oil just dripping or is it coming out pressurized. What exactly is wet? Valve cover gaskets are common on those engines, that is a pretty cheap fix Dealer cost is 20.51. After market parts would be cheaper. The other thing that goes on those is head gaskets. They seem to be good to about 150k. A guy i work with races those engines in a cavalier at a circle track close to me. I have a lot of experience with those under extreme situations. He said that if you are going to do a head gasket and plan on keeping it take a look at the timing chain and the tensioner, its plastic and rarely lasts more than 200k.