Once I read in a newspaper that operating temperature of 80 degrees is a good one for a petrol engine, 85 is even better, considered perfect. Diesel engines have their operating temp 5 degrees higher than petrol engines- that would make 90 degrees.

With our new 280E the operating temp, indeed, was 85 steady. No matter what the driving conditions were, it had 85 degrees. Now 10 years later the temp rises a few degrees but doesn't exceed 90 degree mark (unles driving in stop-and-go traffic, A/C on etc but that is considered normal).

My 250TD (2.5 Turbo) has its operating temp 85 degrees also when driving in a standard way, though the temp climbs happily to 90-95 in hot summer days, even 100 in stop-and-go traffic. I don't consider this to be a problem, diesels run hotter than petrols, 90 is by all means a perfect operating temperature and variations (95 degrees for example) are a part of running the engine under daily conditions.

Mind that a new petrol engine will run 100 degrees and more when the day is hot and your A/C is on and you hardly move on in traffic. You are only to start worrying when the temp does not go down when your cooling aids kick in or when the temp does not go down when you start moving, allowing more cooling air past the radiator.