If you are succeeding in adjusting your oil temp with blank off plates, etc on a cold day, my impression is that you have oil going through the cooler.  This is not the design intent.  It should have no flow or in light of Chris' post, it should have very little flow.

Larry,

 

Even Lycoming recognizes cool oil temps.  In their 320 Operating Handbook, they recommend temps (measured at the standard location) as follows:

 

Ambient

Air (*F)      Oil temperature range (*F)

30 to 90    180 to 245

00 to 70    170 to 225

<10          160 to 210

 

Note, they only reference non-synthetic oils. 

 

The Vernatherm is not a switch, but a temperature controlled flow valve.  It may only completely close off flow at very low temps such as 100F.  Although oil can provide up to 30% of the engine cooling, there are many variables.  The more expensive Stewart-Warner oil coolers are 20% more efficient than its' closest competitor for the same sized cooler.  Remote oil filters can provide some small amount of oil cooling.  Efficient engine cooling in colder air temperatures may not require as much heat exchange thru the oil.  Some aircraft manufacturer's winterization kits partially block engine/cooler air inlets or outlets and may even insulate the crankcase breather tube.

 

I utilize a cockpit controlled air valve at the entry to the oil cooler plenum to maintain minimum temps in the 170 to 180 range. Today, at 4500 ft, 176 KIAS and 40F I had the door closed about 3/4 of the way.  I do have a remote oil filter and I use a semi-synthetic oil I like to keep hot (AeroShell 15W50). 

 

I have never seen moisture at the dipstick in my Lycoming engine (funny metal cap, brass stick and plastic filler tube).  Continentals are another story. I have never seen a Continental engine that did not have a rusty steel dip stick cap.  A friend with a Baron always opens the filler tube after a flight so that the cap can dry out. OK, so I don't see any rust on his caps.