I changed valve covers and gaskets & spark plugs. It's not leaking a whole lot. Only drips. And mechanic said that it doesn't leak while running only after I've driven the car for a while and park it, then the oil drips. Can't seem to find where it's coming from.

 

Could be the Rear Main Seal. Its rare but RMS leaks seems to happen when a car gets over 10 years old.

 

Have you checked the oil cooler O-ring? It's right behind where the oil filter screws on. Have you actually verified that it isn't drippingwhile it's running?

ETA

Check this thread out. Maybe it'll help you out.