I see zero reason to use anything higher than 5W, or you are just increasing cold startup wear.

Higher cold viscosity = longer for oil to flow to the engine internal when it's cold = more wear

Another myth is "thinner oil is less good". If we are talking about driving in Texas or tracking, then yeah, thinner oil will break down from the high temperature, but if you just DD the car somewhere cool like where I live, 5W-20 will do just fine.

I agree on the DD part forsure. I know some will disagree but the Manufacturer tends to know best on those terms.

The issue with running 0w-XX or 5w-XX is that the broader the viscosity range, the more potential there is for the viscosity modifiers to break down.

Use the narrowest range you can reasonably get away with. If you live in a place where the low temperatures don't go below freezing then you can easily use a 10w-XX/15w-XX/20w-XX.

\\\\\oooh an oil thread\\\\\\\

Everywhere else in the world Mazda specifies this range of oil viscosities for the 13-MSP RX-8. 5W20 was forced on many manufactures in the US due to the apparent pittance 0.1-0.2% improvement in fuel economy (I heart the S EPA) using lower viscosity oils. See the chart from the Mazda OEM OM:
https://cimg7.ibsrv.net/gimg/www.rx8...406f5bbb40.jpg

RX-8 ex. US owner's manual recommendation for oil use by temperature range.

Got the car back the day after Christmas with the reman motor. First impressions: no noticeable difference.

I haven't driven it that much though and I'm going through a new motor break-in so I'll see if that changes once I can drive it harder. Also I'm going to get a compression test on the new motor at 1k miles; I'll be sure to share the results here.

Fresh reman/rebuilt motors have low compression until they break in. The break-in process will probably be gradual enough that you won't notice the increase without sensitive instrumentation.

If you live somewhere hot enough to warrant the use of XW-30 or XW-40, I suppose 10W and 15W will be fine. Good point.