I don't have a oil temp gauge installed but every single MB diesel that I have had with an oil cooler it would always work, after a sufficient fully warmed up drive.
I'm saying "it works" by feeling the hoses and the cooler itself. The side/tank of the cooler where the oil enters is "damn hot" and the opposite side would be "much cooler but still warm".
In this 300SDL in question the oil cooler and both its hoses are 100% outside temperature.

Just install a thermo strip on the outside of the canister. Pop the hood after a run and see what the temperature is. If you're not getting more than 195F, then there isn't a problem.

CoolCat Express Corp.: LCD Thermometers 194F-248F (90C-120C)

On Topic Q. ?
 

Looking at the diagram, # 287 is a back flow check valve .

? Does this ever wear out or is it not included on 616 engines ? .

I ask because my 1982 240D's oil pressure always takes longer to come up than does any 617 I've had and I wondered if maybe the oil is draining back into the sump......

? Thoughts ? .

My SDL's oil cooler function depends heavily on driving style, ambient temps, and length of drive.

In cooler weather, or weather that doesn't require A/C, the cooler lines never get hot unless the car has been on the highway for a while.

Hot weather, or heavy A/C usage will get them hot after ~20 min drive with mixed city/highway driving. Longer if all city.

My point is that if the cooling system is healthy, it may take a while for the cooler thermostat to open up. As mentioned above, the oil temps have to be far hotter than the engine coolant temp for the thermostat to open. 8+ quarts of oil and a relatively strong cooling system mean it's gonna take a lot of heat before the oil gets to those kinds of temperatures. Remember that the head and oil sump are aluminum and will cool the oil significantly better than steel or iron will.

Go romp on it hard on a hot day with the A/C on for >30 mins and see if the lines get hot. If they don't, then you should consider working on the oil thermostat.