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.

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87 300SDL
87 190D 2.5NA 5sp
90 300GD with 606 turbo!
08 GL320 CDI
12 ML350 Bluetec
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Past rides:
82 300SD - the one that started it all!
81 240D
87 300SDL - the first one.
92 300D 2.5 Turbo
99 E300 Turbodiesel
07 E320 Bluetec
12 S350 Bluetec

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)

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 ? .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

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.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)