We would speculate that well over half of these oil cooler thermostats are no longer working on these old 77 to 85 Diesel engines, but it is possible to test them in a carefully controlled environment.
More coming Mid-November. For 300D, 300CD, 300TD, and 300SD built in those years. This thermostat has a very limited supply. You might want to purchase while still available. There is NO substitute.
Your diesel should run between 80-85C. It if runs over 95C, it is not healthy for the engine. This might cool it down 5C unless you have major problems like cylinder head crack or leaking head gasket.