The first place I checked was my owner's manual, and it startled me that there was no temp/visc chart and no recommendations other than to ask the dealer. Currently I'm using Chevron Delo 400 15W-40, since The Swamp (aka New Orleans) rarely sees a freeze and temps will soar up into the damp and sticky '90s from now until November. (If we're lucky, the vicious Loozyana summer will end that soon. It could drag on into December.)

I've thought about going to a thinner oil in an attempt to eke out a little more miles per gallon. The temp/visc chart in the .pdf whose link you posted, Michael, seems to indicate that 5W-30 (even 0W-30!) would be OK for temps above 85 F., though apparently MB means only synthetics in those grades.

This of course is a topic that's been discussed to the point of extreme unction around here -- and I flatter myself that I've read almost every thread on the subject -- but I guess it comes down to "Whatever works for you."

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-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino