Diesel coolant temperature running hot

I replaced the thermostat in my '85 300D turbo because the car was running too cold - typically at 70 degrees C, and it would only climb to 80 degrees when sitting in traffic. I put in an OEM MB thermostat rated for 80 degrees C, and refilled the system with yellow MB coolant, and let the car idle with the reservoir tank cap open to purge any air trapped in the system. The temperature gauge climbed to 90 degrees, and after driving the car for a while, to a steady 95 degrees. I let the car sit overnight, added coolant to the reservoir to the proper level, and drove the car at various speeds for about 1/2 hour. The coolant temperature remains steady at 95 degrees C. My '83 300SD with the same engine indicates a steady 90 degrees C. The engine in the 300D does not appear to be running any hotter and has good power. Should I be concerned about this? Is it possible to get a bad thermostat that open at too high a temperature? Should I try another one? Tip for replacing the thermostat: Place a plastic bag over the alternator so that when you unbolt the thermostat housing, the coolant will not drench your alternator. This happens even after you drain the radiator. I already learned that lesson on the 300SD. Is there a coolant drain plug on the engine block?