It's entirely possible the filter isn't tightened properly. Or, it could be the crankcase breather; I have to admit I put 4 full quarts of 20W50 in two weeks ago and no drips and believe me I've pressurized the crankcase more than once since then so I doubt that's the case.

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)