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Do we really need to be concerned about oil temperature extremes?
Only in terms of how those extremes effect our oil selection. More extreme? More important.
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Don't cooling systems keep engine temperatures from exceeding normal operating range?
For coolant? Yes, usually. For oil ..on some level. The oil temp is (mainly) the thermal back pressure of the engine in terms of btu's that aren't absorbed by the cooling jackets ..or sent out the exhaust.
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Parked in the house garage, I doubt our vehicle ever gets below 20 degrees F.
Especially if you live in a region that never sees 20F. How about a unheated shed in North Dakota in February??
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When driven somewhere and parked outside, the engine thermal mass retains heat for a long time.
It depends on the effective radiating surfaces, their thermal conductance, and the differential presented in ambient temp. That is, YMMV.
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By the time we leave from a store, etc., the engine has not cooled that much.
Depends on how long you're in the store ..how long it was to get there ...how cold it is (see above).
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Does an engine really ever experience the extremes for which oils are supposedly formulated?
Well, what do you mean by "formulated (for)" (the infamous "Well, first I think we need to define "completely destroyed" before I can comment")? When you're on the highway for a long enough period ..you'll reach the 200-220F temp range (probably). It will vary somewhat in non-exchanged/forced cooled engines depending on load and ambient temp.
If you mean hit 280-302F to test our HTHS ratings?? Not usually (unless racing or climbing some 8% grade for a long time at high load, as was mentioned).
As Dr. Haas explains ..coolant is quick to heat and is regulated to maintain a max temp limit in a narrow range. Oil typically is not and takes a long time to peak out at a normalized temp indexed for the load and for the ambient temperature in non-exchanged/forced cooled engines.