If you have an oil leak at the filter, you wont see it under the car due to the engine sheild at the bottom. Rub your hand around the filter to see if your loosing anything. If not, remove the bottom plastic sheild and see if any fresh oil has gathered. If so, you should see roughly where its coming from. I have absolutely no leaks on my car but do have to top it up very little (about 2mm on the stick) just to keep the levels right, about every 45 to 60 days. How often you checking?