My first post: this must be addressed somewhere in the old threads but I couldn't find it. My 82 300SD (617?) is running at about 78C. Now, I'm in Massachusetts so it's about 35F outside and I have the heat on. Turning the heat off only brings it up to 80C. I remember in the summer, the car usually ran at 90C with no AC on. Is it ok for the running temp to be so affected by the weather and heater usage and is 78C just too low for good economy and engine protection?

I'm mainly asking because this is my first winter with the car (but not my first Mercedes, previously 1967 200D). I bought this car knowing I was inheriting someone else's problems so I count on everything needing to be replaced. I know I could just go ahead and replace the thermostat but I want to take a break because I've already done so many other things lately (tie rods, center link, valve adjust, ALL filters, oil cooler hoses, seat switches, air cleaner brackets, steering stabilizers, auxilliary fan, etc.) Thanks!