You don't have a problem. Summer is upon us and temperatures in stop and go traffic will increase some. 90 degrees Centigrade is nothing to worry about unless you're losing coolant for some reason.
I believe that good, clean tapwater and antifreeze mixed 50-50 is a safe bet.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in