I do wonder if Mazda has any problem using Group III synthetics in the rotaries? Or if they were just talking about PAO based oils?
I am not a raotary expert but I have heard of two issues and can think of a third. The first is that synthetics do not burn as easily as petroleum based oils and the Renesis engine injects oil into the combustion process. The second is that a film is left once the synthetics are inefficiently burned in the combustion process and that has a detrimental effect. The main issue for me would be the avoiding of my warranty and Mazda is not shy about doing that.
If you are getting "carbon/gum" buildup, +1 on low ash (<0.9%) and I would also recommend low VII content which a 5w-20 would have and a real synth grp iv/v blend would/should have none. Gee why didnt they put in a tank you could fill with TWC3 outboard oil? These oils have ~1/2 the flash of conventional motor oils.
I have had 3 RX7's. In street driven applications, regular dino low-ash 5w-20 or 5w-30 should be fine. You can even use the hydrocracked Motorcraft stuff if you'd like if you have the absolute need to use a synthetic. Otherwise, for racing applications and sustained high-rpm, you can use synthetic low-ash oils.
Some feel that the 5w-20 was too thin for the RX-8 and had side seal and housing failures; using thick oils does not have any negative effects in a rotary, the "crankshaft" portion is partially gear-driven and does not like thin stuff, and you also won't see any horsepower/mileage losses in going to thicker stuff in a rotary, in general.
In my street rotaries, I used a low-ash HDEO 15w-40 diesel oil. In the racecars, same stuff or Valvoline VR1 20w-50.
I just finished reading it, seems like Amsoil's statement is a "we are innocent until proven guilty" defensive statement. If I were a rotary owner I won't feel comfortable trusting it.
Now if Amsoil come out with a long term engine test with tear down, then they would earn my trust.
Is this "Mazda genuine oil"?
I was trying to apply Mazda rotary issues to all gasoline engines. I think Mazda genuine oil is Conoco/Motorcraft syn blend because I couldn't find any other info using key word searches. And I don't think the 5w-30 is a good grade for the rotary because in UOA it shears too much. On other designs deposits could go to ring packs.