Except on the 116 chassis, the 617.95 oil thermostat begins to open at 110 degrees C and fully opens at 125 degrees C. (Keep in mind that is oil temp, not coolant temp.)

Depends on whether the oil is single grade or multigrade.

Negative - on 123 the oil cooler opens @ 95c and is fully open @ 110c

http://rbk3.com/mbz/tsm/MBZ18-120%20OIL%20THERMO.pdf

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Cheers,
Robert

Since not all 123's have the same engine, maybe we need to get specific about the exact engine model. My figures came from the MB 617.95 service manual (page 18.8-005/5)
The subject vehicle, an '81 300TD, should have a 617.952 engine.

Keep in mind that the temperatures noted for oil thermostat opening are oil temperature, not coolant temp.

With that in mind, I would think any of the turbocharged engines would use their oil coolers almost all the time, regardless of climate, because the turbo puts a lot of heat into the oil, and the oil also functions as a coolant for the turbo.

As a side note, I've also noticed my oil consumption seems to increase when my routine is mostly city driving. In other words, my miles-per-quart for oil consumption seems to be greater in city driving, if that makes any sense. This would support coldwar's theory about oil lingering in the upper cylinders until the engine is really hot (though I'm not sure what you mean by "upper cylinders").

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'81 300TD turbo <<sold 12/05 to another diesel nut

I don't know squat about the TDs, I guess thats obvious.

__________________
Cheers,
Robert