Doug,

 

Since the vernatherm is a oil "thermostat", it makes sense that even in really cold weather, the vernatherm should regulate the oil temp to 180.  The problem comes from the bypass circuit.  These coolers have a bypass circuit that keeps oil from congealing in the cooler when the flow is stopped by the vernatherm.  A fair amount of oil can still circulate through the cooler even when the vernatherm is below 180.  The oil door (or a Cherokee style baffle plate) will help keep the bypass oil from cooling off.

 

The stock TCM IO-550N comes with a different cooler than the "G" or the TSIO.  It has a different design bypass circuit.  Since your engine is custom built, you may have the "cool" cooler.  Some of the "hot" coolers may be suffering from a leak in the bypass circuit that allows oil to flow through the bypass circuit instead of through the cooling fins, reducing efficiency.

 

Mike Easley

Colorado Springs