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The most debated question in the community, bordering on a religious war, so I won't try to cover every point here.
It boils down to deciding for yourself.
Some go with 5w20, to stay with Mazda's recommendation in North America. Some go with 5w30 to go with Mazda's recommendation outside North America. Others go to 10w40 or even 20w50, the most common weights among RX-7s. The general argument revolves around if 5w20 is too thin or not. Typically, you want to go with a heavier weight oil (higher numbers) the hotter the environment you live in. Many manuals outside of North America state something to this effect. So research up on it, and make your own decision.
Dino vs synthetic is another hot topic, with the main argument revolving around if the strength of the synthetic is worth the cost, and if there is any better burning (or not better burning) than dino oils, and if any deposits are left behind that could increase engine wear. Mazda only recommends non-synthetic, but does not require it. Again, your decision.
And yes, all weights of oils mix with all other weights of oils, roughly averaging the numbers. Half 5w20 and half 5w40 is roughly 5w30, for example. Not precisely, but close enough. Dino also mixes entirely fine with synthetic.
Here is another perspective...
It doesn't really matter
Fresh oil on regular changes is far far far more important than ANY of the above attributes. And the only impact that crankcase oil type and viscosity lubrication has on your engine's lifespan is bearing wear, which is not a severe concern for us, since we have numerous other methods of engine failure that have nothing to do with oil attributes!
Outside of lubricating the e-shaft bearings and being available for injection into the combustion chamber (where none of the oil's attributes matter at all), the oil's benefit is a secondary method of assisting to keep our engines cool. Oil is fantastic for transporting heat out of the engine. And all types and viscosity do that equally well, or as close to equally as won't make a tangible difference.
Well, are you really going to argue that filthy dirty 4-stroke dino oil burns cleaner than filthy dirty 4-stroke synthetic oil? NEITHER of them were meant to be burned, and crankcase oil is very very filthy. If you are so concerned about how clean burning your oil is, then get a SOHN adapter and start burning clean 2-stroke! Then you can pick the oil that you want for the high stress, high rpm, high sheer environment of the e-shaft bearings. Besides that though, even dirty 4-stroke burns cleaner than gasoline. If you are really serious about how cleanly the fluids in your engine burns, switch to a fuel that doesn't carbon cake the engine.
So if oil weight matters to you, then do some reading on the various opinions, find an opinion you somewhat agree with that is made in reference to the climate you drive in, the drive style you do, and your budget, and go with that.
There is a fairly limited amount of actual test data about oil. What we do have is used oil analysis tests, by companies like Blackstone Labs. These tests measure the amount of contaminates in the oil, things like water, gasoline, wear metals, sulfur, etc... They give a pretty good idea of how well the oil you are sampling protected your engine. The discussion thread about the tests is here: RX8Club.com: Used Oil Analysis thread