There's definitely an answer to it on the board somewhere, I'll have to do a search to find it though.
Basically though moisture does get lost throughout the day depending on your hairs condition.
Using oil over a moisterizer does help to seal in that moister for a finite number of hours but, it does evaporate over time.
For some, the moister and sealing takes place on a daily basis, some don't do it so often which can also change when the seasons change.
Like for me, I find I need more moisturizer in the summer/winter than in the spring/fall. But someone else may not be on that same schedule.
I'm also learning that as my hair gets longer my needs are changing, especially where I put extra moisturizer.
I'll see what else I can dig up cause its around here somewhere.
-A
Found it!
http://www.longhaircareforum.com/showthread.php?t=84746 Midway through the first message from Sistaslick