Rebuilt motor

Ok, recently put a motor together, not truly a rebuild, but a tight motor. When
I first start out with it, it runs fine, but after about 5 - 8 miles, it starts
to puff smoke, then after a few more miles it will blow smoke. I would think if
it was a ring or head problem, it would do it at the get go. This is making me
think that with the engine warming, the oil is thinning, and perhaps my PCV
valve is ruptured, allowing the engine to draw in oil from the timing cover.
Does this make sense, or not? Any other thoughts?