There are a millions things that can cause it, oil rings?!!!, stem seals, intake manifold leak, and few hundred other things. I would check the obvious first and see if the intake manifold has a leak in the lifter valley and might be sucking the oil from there.
There certainly seems to be a lot of sucking around here these days. <img src="graemlins/mwink.gif" border="0" alt="[mwink]" />
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo