The most common area on an engine to develop a oil leak is the valve cover. They are epecially prone to leak in the winter when it is cold out. You may notice it lessen as we go into spring/summer. The oil can make a real mess as it runs down the side of the engine and forms a sludge. If it is leaking a lot get it repaired, however ,if it is just a small nuisance, they usually are harmless. Can also be a filter leaking. Causes of a filter leaking are, too loose, too tight, or gasket problem. The filter gasket should be coated with oil before installation. Filter should be tightened 3/4 turn after making contact with the mounting surface. No more, no less. Also if the old gasket was left on, then the new filter will leak.