When I got the car, 128K miles, in 2005, it had Mobil-1, and I continued that.

I used 5W-40, in order to have rapid pump-up of the valves when starting cold, and to have better operating temp protection.

Now it is hot hot hot here (30 days over 100F), and so I am considering changing the weight. My indie shop asked why I went to 10W-40 on my S/90. He said the fine clearances on these engines (B6304) need a better top-end weight. So, I looked in the Owner's Manual. Confusion - at least for me.

The Book says:

SAE 5W-30 is OK from below -22F to 86F.

SAE 5W-40 is OK from below -22F to 104F.

SAE 10W-30 is OK from -4F to 104F.

The 5W-40 is getting hard to find, impossible if out on the road somewhere, such a Pecos, TX. Looks like the easy-to-find 10W-30 would be OK for our current climate,

But how can it "reach" to 104F? Does the lesser "span" of viscosity make the difference? Will it be as thin as the 5W-40 at operating temps? Does it have more oil and less "viscosity stretchers" than does the 5W-40?

Both the 965 and my S/90 loose about a quart over a 5K oil change interval, would use of the 10W-30 maybe reduce that usage a bit?

Your answers and thoughts willbe appreciated.

Bob

:>)