Thank you all for forging through all of this.
Living in N. California, where the temps can range quite a bit-(Sierras in the winter to the valleys in mid-summer), I too have watched the temp gauges on my diesels (past cars) and with the current ones ('91 300E & '95 E320 Wagon) and wondered what was OK and what was not.
I have driven through Death Valley during a "mild" summer where it was "only" 129 in the old '82 240D. I have also driven through a -15 night in the high peaks. What I have always come back to was what Mercedes recommends.
I have been educated through your comments, research and postings. I was never quite sure where MBZ's temperatures were supposed to be.
Thanks again for this.