[ovilla]

Go to your nearest FLAPS and get some "Simple Green" spray or any other degreaser or brake cleaner. Simple Green is not strong and you'll be able to start cleaning up right away. Also, replace the oil pressure switch since these things always seem to leak. I've even had a brand new one start leaking on me. I would also replace the valve cover gaskets as these can shrink with time and also leak (even upwards, not just downwards). Get a new oil filter too and make sure you wipe down the mating surface of the filter before you put it on. Finally, when all is clean, put the rear of the car up on ramps, chock the front wheels and have someone turn it on for just a minute or two and then turn it off. Since you started with a really clean engine you should now be able to feel the oil with your fingertips (or a clean rag should be able to find traces of oil).
Finally, if you still can't figure out what's going on try this. Go to your local hardware store and buy a "black light" bulb. Then call around to your FLAPS and tell them that you want some oil dye to find an oil leak. This stuff works great. Clean your engine again and then put some of this dye in your engine. Then have someone start the engine and you hold up your black light against the bottom of the engine. The oil dye will make your oil leaks look like thick runs of white toothpaste running down the engine block. To see it better, close your garage door and dim your garage lights really low. Good luck.