Could wellbe the CCV system blocked causing pressure to build up and force oil past the oil filler cap gasket. If its in the plug wells then suspect the centre plug well gasket under the camshaft cover, they go hard with age and heat cycles and eventually go brittle and crack causing oil to leak into the plug wells. Always fit new main camshaft cover gasket at the same time, they sell gaskets in a kit which includes both gaskets. Depending on model you need to replace the cam cover round rubber seals under the cover retaining bolts as well. Did this on my 2.5 plug well gaskets were very brittle but hadn't developed any leaks like the main gasket had causing oil to drop on the exhaust and smoke. It was only a matter of time before the plug well gaskets let go. Check the condition of the rubber seal under the oil cap, if its bad replace as suggested earlier in this thread. Pull the CCV pipes and see if there is a build up of oily gunge in them, if there is might be time to replace with new kit.