[SpeZ]

At both coolers, inlet is bottom side so its got better chance to push air out.
Connecting two coolers in series would increase resistance I believe, but definetly would lower the cooling.
Pre cooled oil would oil flow in the second oil cooler and the cooling power is reduced by less Delta. (Difference between ambient and oil is less, so its cooling effect.)

I really appreciate your input and to exchange thoughts and sights.

But having in both coolers nearly no oil shows that there is really ~50% less flow and reaching pretty much laminar flow. By this reason it only flows through a few cooler rows.
But once they're pre filled I am sure the cooling benefit is massive.
The benefit of 50% less oil velocity, the oil pressure loss in hoses, full bent high flow fittings is very small.
Yes, this might also decrease cooling power inside the cooler by a little.

I will keep you all updated.