Engine oil leaks not only increase engine friction, but those under-the-hood leaks (particularly at the valve covers) allow oil to seep onto hoses, belts and wiring harnesses, causing them to deteriorate. Periodically tighten the screws holding all oil-sealing covers, like cam or pushrod gallery covers. If a washer is the specially shaped load-spreading type and it's missing, get a replacement that matches the original, as it significantly improves sealing over an ordinary washer.
Some OEM-type gaskets have been superceded by a newer style because the originals were -you guessed it- prone to leak. So be sure you're buying the newest revision. Also check the oil-pressure sensing unit for seepage, and if necessary, remove the unit and wrap the threads with Teflon tape for a better seal.