my 190e2.6 also likes to run hot. That is, during the summer, if you leave the car idling, the fans will kick in when the temp needle gets past that 3rd mark on the gauge. And it will stay there for quite a few minutes.
definitely, there is a sweet spot for the operating temperature of the engine, and it is not only the combustion efficiency that is affected by the temperature.
the effects of temperature on the engine oil are also important. With the engine idling, notice that the oil pressure gauge will show a lower reading when the oil is hot, than when it is cold. This is because the oil viscosity is low at elevated temps, and the effectivity of the oil pump is affected. If the oil viscosity is too low, the oil film that forms in the journal bearings may be too thin, risking metal-to-metal contact at high loads.
if the oil viscosity is too high (e.g. at low temperatures), the oil may not flow freely enough within the engine.
At the extreme, if the oil temp is high enough, the oil can also carbonize (but this will be when other unspeakable things have already happened to your engine ... at which point the oil is the least of your problems ...).
in the winter here in canada, my car wants to be warmed up thoroughly before it can be driven decently. Seems like there are other things upstream of the engine, in the induction and fuel delivery systems that have a sweet spot as well.