My first thought would also be the filter. It could also be a super slow leak from aging gaskets, which (if you want to go the "don't get your hands dirty route") could possibly be solved with an OTC conditioner product like this - Oil Stop Leak | Engine Oil Leak Sealant | BlueDevil. These additives drive some people nuts, but they have their place. If it doesn't work you're only out a few bucks anyhow. If it works it's a cheap fix that saves you some time.