What engine does it have in it?
If it's the 3.1 V6, they have problems leaking oil from the top of the oil pump drive shaft (where the distributor would be if it had one).
Look under the intake manifold, above the thermostat. It is hard to see, but I bet you it's leaking from there. The gasket is only $4 or so to replace, but you have a LOT of stuff to take off to get at it.
Sometimes oil will leak down all over the engine and not drop much on the ground because the oil only leaks with the engine running. At least that was my experience. I was adding a quart every 400 miles until I figured out where it was coming from!
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