Al,
I am on travel and getting intermittent messages, but have you tried energizing the boundary layer ahead of the entrance to your oil cooler.

Local builder found this very effective on a duct that wasn't working well because it was getting boundary layer feed.

a couple of vortex generators ahead of the entrance. If you have tried this already, I may have missed the message.

Also, he found a duct separator (turning vane) like you drew helped in another case.

Bill schertz