There are two different valve covers out there, a cast aluminum and a plastic valve cover.
I don't know about the plastic ones. I have the cast aluminum one and it had the gray rubber looking crap and it leak profusely when I bought my jeep.
With many engines I have had good luck with the felpro cork rubber valve cover gaskets. This is what I used on mine. I clean the old gasket off and use a scothbrite pad with brake cleaner to clean the surface.
Then I use either weatherstrip adhesive or Yamabond(i love that stuff) to glue the gasket to the valve cover and let it tack up real good. I grease the side of the gasket touching the head. Then torque it down to specs evenly, which on the 2.5 is 55 inch pounds. Remember to retorque the cover as it will settle over time. I had to do it 4 times on my jeep before it stopped settling. By greasing the one side you can remove the cover in the future without needing to buy a new gasket. This is especially important on engines with solid lifters so you can adjust the valves, which shouldn't be a problem with the jeep. I have never had a leak using the felpro cork rubber gaskets on many chevy V8's, several allis chalmers tractors and currently my jeep, Dry as a bone. Saves money as you don't need to buy a new gasket every time or reapply silicone.
I HATE silicone because as a mechanic its hell to clean up, looks like hell, and is rarely applied correctly by most folks. If you do use silicone remember to follow the directions and just set the cover on the engine and let it skin up for an hour or so before torquing down. I have seen silicone leak many times because this direction wasn't followed and the silicone squeezed out in places.
