if you have an 80 tstat and the coolant still falls below 70C while driving your thermostat is faulty. A 5 to 10 degree difference is normal for an NA diesel that is a lazy one like the 617, the 617 turbo pushes hard and usually keeps its temperature at 80C
my 606 has an 85 tstat, the temperature is always at 90C, this m112 engine (320 v6) has an 87C stat but keeps the engine at 82(ish) while driving as the system is a bit oversized and very efficient compared to the 606 engine which has a mile long 6 cylinder block and a very tiny pump (it was tiny to reduce power loss) - its quite a thought when you see MB engine design. Everything is simple looking but is very carefully put together. The other bit is that the cooling fan and shroud is a daft design.
Regardless both engines get to 98~100C at a red traffic light.
