My bike does exactly the same thing. Within a week all the oil from the tank ends up in the crankcase. I heard that keeping the engine on compression stroke when parked can help, but I tried this with no tangible results. I have also heard that using straight 50W oil can help, but haven't tried this yet.
My bike has a new lower end in the engine, so the most likely culprit is excessive clearance between the oil pump gears and the oil pump side plates. My research has found that you can reduce this clearance by disassembling the oil pump and lapping the side plates. The factory manual has some instructions on this.
Another option is to have the Nortech anti-wet sumping modification done on your timing cover. This is done by a company named AMR in Tuscon Arizona (www.amr-of-tucson.com/nortech.html). This url will give you more information. You have to send in your oil pump and timing cover for the mods. As part of this service, they refurbish the oil pump to reduce the clearances, and also install a valve in the timing cover. I called AMR and spoke with the owner about the mod and I plan on getting it done to my bike as soon as I can get some time to pull off the timing cover.
Anyone out there have any experience with this Nortech mod?