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Experimentation is the mother of truth.

The thermostatic valve in a Lycoming 320/360 (Vernatherm) never completely closes the oil path to the cooler (See Gary Rodgers comments, LML V1 #2).  Examination of the Vernatherm shows that it is a plunger whose length is controlled by temperature. This is a continuous operation beginning from some low temperature (maybe 80F) to fully open at about 180F.  The oil faucet to the cooler is always open to some degree (pun intended) until full flow at 180F.

Cruise operation in dense cold air (not the hi-altitude thin cold air), where true airspeed can be less than indicated and engine cooling is great, results in oil temps that may not rise above 150F.  This can be problematic because combustion byproducts and water are not cooked out of the oil.    Observation has shown that a winter oil change followed by low oil temp operation results in the oil darkening in a few hours.  Summer changes and operation at higher oil temps (180F-200F) keeps the oil looking fresher, longer.

I recently installed an air door on the oil cooler air inlet (instrument panel knob - cable contolled) which has given me complete control over the oil temps.  On cold winter days the air door is almost totally closed and the oil temp is maintained at 180 degrees.

Scott Krueger N92EX

P.S.  Gary, I live in the Chicago area and we blame Canada for our cold air masses.  Who do you blame?

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Please send your photos and drawings to marvkaye@olsusa.com.