Mercedes-Benz: 560SEL temp control

I bought a '89 560SEL (64K mi.) a couple of months ago. My first MB. I am
really enjoying it. Yesterday was the first time I have gotten a chance to
get it out on the open road, as well as a few back country roads, and push
it a a bit. Some MB indies I spoke to said to use 3rd gear for all common
driving until I approach 100mph, then go to "D". Here in the Los Angeles
area its tough to find any roadways appropriate for this type of speed, so I
don't imagine needing the "D" any time soon. My question has to do with
controlling the coolant temp. I was doing about 80 mph and hit an uphill
grade. Dodging other traffic caused me to slow down, then accelerate again.
Love the power to spare, but the coolant temp had been steady at about 90C
but then crept up steadily to about 110C in about one minute. This rise
concerned me so I backed off the power and the temp came right down.
Shifted to "D" and temp came down more. I am not used to temps doing this
in a large engine. Is this common behavior? At what temp would the rest of
you be concerned?

Extra data: The hill climb event was done with AC running. This had a
significant effect on temps.
I had the radiator replaced when I bought the car.

TIA

Jack

Its normal for the engine's temperature to rise when climbing a hill as
more fuel is consumed and that means more heat needs to be shed.
Acceleration and hill climbing consume the most fuel; acceleration is
usually too brief for the driver to notice a rise in the engine's
temperature.

As to driving in third gear - don't. This engine has lots of torque to
push even this relatively heavy car along in fourth gear so use it.
You'll always find lots of "experts" who will tell you plain B.S. about
how these cars should be driven, serviced and repaired. Follow the M-B
owner's book not the "advice" of some moron!

You're lucky to have found such a low mileage 560SEL, take good care of
it with regular maintenance and it will serve you well. Nice car.