With the return below the oil level in the pan, obviously the pan oil will back up into the drain line.  I think Tracy made the key point.  If the oil is from the re-drive is “frothy” (which it likely is) it will “float” at the interface with the clear oil and back up the drain line; in other words, not drain.

This is a pretty good description Al.  For some reason, I’ve always thought there would be some oil pooled up in the redrive, but now that I think about it, I’m not so sure.  If there was a pool of oil, it would stand to reason that the frothy stuff would stay at the top, and the clear oil would be at the bottom, where the drain lines are.  Does anyone know if the drive does stay partially full when running? 

Cheers,

Rusty