Maybe the intake gaskets that love to fail on those,(assuming it's a 60 degree 3.1 or 3.4), is seaping slightly into oil, and is burning out on it's own. or it's leaking externally and evaporating on the outside. It's going somewhere, and it's new enough that the block most likely is not cracked through the cylinders like some late eighties-early nineties 2.8's and 3.1's. Not likely.