I agree with Larry, I use Chevron Delo 15-40 all year round, but we don't get temp extremes here in the pacific northwest.

Years ago I would switch to Valvoline 20-50 in the summer months, but don't do that now because the change in oil formulation. With high miles and really hot weather you could probably use Valvoline, but you would have to add a zinc supplement. I think GM EOS (engine oil supplement) an engine break in product will fit the bill and is probably available at GM dealers now ( it was taken off the market for a while I heard).

Be sure not to overfill either. I have now switched to sucking the old oil thru the dip stick like you would with a boat, much easier on the back.

__________________
1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)