2.5L engine oil leak

Got another problem with my 91 Grand Am with 2.5L engine. I got an engine out of the junkyard and everything looked ok. It had been washed off and I could see no leak problems. When I started it and it got up to operating temperature, it had a consistent drip from the back of the oil pan. I figured I'd been dumb trying to save a buck and not just installing a new rear main seal when I had the chance, so pulled it out and installed one, and did not notice any wear on the crankshaft. As a precaution, I pulled off the plate covering the back of the camshaft, and made a thin paper seal when I could not find one to buy, and dumped the engine back in. I cranked it up and it still leaked. I then really got to looking at it while it was running and really could not see the oil drip coming from where I thought it would be. I took a chance and pulled the pan, found a suspect place on it, cleaned it up, put RTV Blue on it and put it back together. I cranked it up the next day and thought I had solved the problem as it did not leak idling in the garage or driving around town. A few days later we went out of town and found it left a spots and got the underside wet with oil. After driving it about 1000 miles so far, it looks to have leaked a half quart of oil and only does so when driving at highway speeds. Again I can't really see exactly where it is coming from except the rear area of the engine. So finally, here's my questions, 1). What is anyone's best guess/probability of where it is leaking from? 2.) The camplate looked to be sealed originally with something that looked like a thin rubber gasket like that on a Mason jar lid. Did I screw up by making a paper gasket? 3.) Is there any special sealant that should be used on this or any of the other surfaces? Any ideas would be greatly appreciated! I don't really want to pull the engine again unless got a good idea this is the problem or maybe I didn't get a good enough seal on the pan? . thanks everyone for the help in advance!

I would first start with a bath for the motor, clean it up so you can see where it isn't leaking. The seal for that plate covering the back of the cam is available at the crooks (dealership), they will probably have to order it for you. I would start with that, then look at your oil pan as it might have a small crack in it. Also, isn't there a knock sensor on the back of the motor on the 2.5? That might be leaking as well. Good luck, oil leaks can be difficult to track down. I did one on a nissan a few years back, turned out to be the fuel pump.