Thank you for you research.
I have to say that driving with 10W-30 in the summer just gives me a bad feeling (very unscientific!) It seems too thin.
The GM concern regarding rapid polymer deterioration in the 10W-40 oils is something I have heard repeatedly, even before seeing it here.
I am thinking of going either to a 20W50 or 15W-40 .....
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)