at guitar stores you might find a product called "finger ease" it's a spray that lubricates the strings and neck so that the strings don't squeak when one changes fret positions and the thumb doesn't stick to the back. but Dave's right the oil from your thumb will polish the neck smooth. try going two-three weeks where you never play less than 4 consecutive hours a day. It takes a bit of time for your hands to start sweating so it's only after you get going that any real polish gets applied to the neck. Also, if you're playing that much your hands are getting used to the paint on the neck, so that extended time of practice has a compounded effect. 6 hours is even better. Not to mention that, if you play that long every day for a few weeks you can actually notice the improvement from one day to the next.