No external leaks means that the it must be leaking into and oil passage or into a cylinder. If you are filling up that often you SHOULD be producing a considerable amount of white smoke due to the burning off of water/coolant.
Pull your spark plugs and check your oil. If your oil is milky, its draining into the engine due to a failed headgasket. This will cause catastrophic bearing failure in short time. It may worth simply draining your oil. If its clean, you can reuse the same oil. I would recommend placing a coffee filter in your funnel to catch any contaminants that may have been in your oil pan prior.