On one car I owned, I switched to 10w40 because with 10w30, my oil pressure dropped <i>very</i> low on hot summer days when idling in traffic. The 10w40 cured the problem, and gave me a few extra psi of safety margin under the same conditions.
For several OCI's, I went from 5/10w30 synthetic blends to Lubro-Moly synthetic 0w40(Full PAO German oil, SJ)- does that count? It held up well, after a long interval it had *barely* sheared into the 30W range. To see the analysis report try a "way-back" search under my membership number in the UOA section. Seemed to cause some "drag", maybe reduce power a little- but I have nothing but the ol' b*tt-dyno to base that on. Gas mileage did drop by a fairly consistent 1 mpg as I recall, about 3% on my car.
Conventional wisdom says that if all else is equal, a 10w30 should be more shear-stable than a 5w30. That may make less difference now than it once did. Remember that oil shears & thins first, & then eventually begins to thicken with further use. If you're concerned with keeping a conventional oil at a 30W for as long as possible, then using a 10w40 is at least worth a try. But to *really know* what's going on with shearing & viscosity, you'd need to have a sample analyzed every 500 miles or so. It would be expensive- but it sure would be interesting!
Yep, even did it when the vehicle was new.
Have a '97 Rodeo that calls for nothing less then 10w-30 in summer heat; but when I lived in Las Vegas, NV - hot desert and did vast amounts of highway driving, that 10w-30 would thin out to nothing. So I started running 10w-40 at barely 12k miles in the summer.
Currently have 155k miles, still likes 10w-40 in the summer.
I was running 5w30 in my '95 F150 4.9L and a few years ago installed an oil pressure gauge. When I saw the pressure was running below the spec range, I switched to 10w40 which put the pressure in the lower end of the spec range. During winter I can still run 10w30, but in summer the pressure can drop a lot.
I did not notice any performance/power difference.
yes. I didn't really use a 10-40, but the HM 10/30's i used had equal or higher viscosities, I assume.
I didn't notice any added power, but the engine idled smoother, quieter, and used less oil, and helped to stop usage completely (coupled with VSOT)