With the cooler in the return line to the tank, it's coming from the scavenge pump section of the oil pump. I don't know the capability of this pump section, but it would normally have very little back pressure since it pumps into a short open line to the oil tank - and a fairly large diameter oil line at that. I don't know that putting in a restrictive cooler would cause a problem.
The problem would be serious - potential engine destruction - if the scavenge volume was reduced to below the main pump volume. The oil tank would run dry (sump filling up as well) and lubricating flow would drop to whatever the scavenge flow was: likely not enough to maintain pressure for proper main bearing lubrication.
This may or may not be an issue depending on your scavenge pump and just how restrictive the cooler and plumbing is. Try it and let us know how it works out?
