In the Corvette forums linked above I recall seeing something about the oil breaking down when it has such a disparity like 0-40 as opposed to a smaller temp range of 0-20.

Larry, if you're still using 0w-20 Mobil1 through the summer months, that's good enough for me...I was just curious if anyone had any insight into that 0-40 weight stuff.

Thanks all for the replies.