Just my opinion but 30w oil seems too light for summer driving.
Have you looked in your manual at the temperature range chart for the various oil weights?
I seem to recall that when temps get to 85f or greater a 40w is called for. But then again I am not as familiar with your car's engine.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)