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 the best and most logical explanation of this that I've heard yet. I completely agree with this. One thing I like about this group is that most members are not satisfied with advice not accompanied with logic. In other words we want to know why we are doing it one way or another. Unlike most of the Lyc installations where tradition & myth govern the installation.

S. Todd Bartrim
Turbo 13B rotary powered
RX-9endurance (FWF)
C-FSTB (reserved)
http://www3.telus.net/haywire/RV-9/C-FSTB.htm

        "Imagination is more important than knowledge"
                                        -Albert Einstein