---- Steven Johnson <sjohnso2000@GMAIL.COM> wrote: 
> I believe that is why we use the multi-weight oils to handle the extremes.

That is true.  However, single weight oil is still considered more stable at high temperatures.  Synthetic is considered more stable than conventional of the same grade.  mcneely

Looking at the figure on page 98 of my owner's book, there may be different ways to interpret the use of single grade vs. multi-grade.  Under single grade, SAE 30 is recommeded at 90 F and below, down to 30 F.  SAE 40 is recommended at 70 F and above, including above 100 F.  Under multi-grade, SAE 15W-40, 15W-50 are recommended at 0 F and above, including above 95 F.  SAE 20W-40 and 20W-50 are recommended at 10 F and above, including above 95 F.  How high the little arrows are intended to go is unclear to me.  I take a rectangle with no triangular end to mean, "don't exceed that temperature with that viscosity."  The little triangular end on the rectangle I have always taken to mean "and above," without worrying about how much above.  Perhaps that is incorrect, given that the little triangular end on the rectangle is at a lower temperature for multi-grades than for single grades.  Hmmmm.  I run my vehicle regularly in summer at ambient temperatures well above 95, even well above 100 F.  Maybe time to go back to the single grade?  Think I might try it.  mcneely