I have a Lycoming O-320 with a remote mounted cooler (on the firewall).

Steve,

 

I am running an IO320 with a 7 vane (small) Stewart-Warner cooler mounted on the left firewall and fed by a large NACA duct on the side of the cowl with the heated air exiting into the lower cowl.  In temps below 20*F, I must partially close off the air to the cooler to keep the oil temp over 170*F. 

 

I typically see CHTs in the 390-400 range (hi-power cruise) and oil right at 180*F.  Oil temps may rise to 190 in max power climbs and will rise to 200-210 in hi-power, lo-speed operation (in the pattern).  I do not see these temps as a problem.

 

Your oil temp sensor should be next to the Vernatherm.

 

I have a friend with a COZY IV that runs an IO360 with cool CHTs and a hot oil problem.  The symptom is a slowly rising oil temp in cruise - indicating that there is not quite enough cooling even though he has measured an adequate 5.5" H2O air pressure across the cooler.  He is switching from a cheaper off-brand cooler to the same size Stewart-Warner because his research shows that the S-W is 20% more efficient than the off-brand.  I will report his results - probably by the end of this weekend's testing.