When the engine is running the cam covers are maintained under engine suction by the breather system. Any leaks in the cam cover gaskets allow air to leak in, not oil to leak out. After many years service the small breather hole blocks, then the cam covers are pressurised by crankcase gas blowby, this pressure eventually blows a hole in the cam cover gasket.
Relieving the blockage in the small breather should rectify matters, though changing the blowing gasket is also advisable. Changing the other cam cover gasket is pointless, though often recommended. Fitting expensive Vx gaskets is also pointless, any old set will do. The Omega 2.6 I bought in May 2015 had a leaking cam cover gasket and a complete set of Leader gaskets in the boot, with invoice for £15.75. I fitted one set to the leaking side, cleared the breather, and handed the car over to my daughter. 18 months and 10267 miles later the car is home again for its MOT, and there are no oil leaks.