G-05 and new thermostat/'88 300SEL
 

I would strongly recommend replacing the thermostat when replacing the coolant. My temp gauge was in the area of 80C and I thought it was OK . Not true!, the car needs 87C and that 7C makes all the difference in the operation of the car.

You are correct. Specs on a properly operating thermostat start to open at 87C and is fully open at 102C. As they get older, they gradually start to open at lower and lower temps. A new thermostat will bring the engine back up to a bottom line of 85-90C on the highway, where it belongs.