Where is the oil coming from?
 

I have a 2002 Cadillac Deville, with a V8 Northstar engine, with around 110,000 miles on it. Over the past two weeks I have noticed an oil leak, mostly from the right side but now more in the middle of the car. Starting around six months ago the car began to start using oil. However, I never noticed a leak. It doesn't smoke nor do I smell any oil. Twice over the past six months the "low oil" light came on and I discovered it was two quarts low each time. I refilled it and the light went off and the car has always run fine. A friend of mine said it was the valve covers leaking. He replaced the gaskets. Twice. It STILL LEAKS. However, it ONLY seems to leak once the motor is hot and once you turn the engine OFF. It does not seem to leak at all while the motor is running. When it does leak it last around ten minutes or so then stops. And, ONLY when it is OFF. The leak amounts to a puddle maybe three or four inches in diameter. Any ideas what is causing it and how to stop it? Thanks for any thought you might have.