"The Mazda factory does not recommend the use of synthetic oils in their rotary engines - specifically addressing this issue in the Owner's Manual.

In 1979, Racing Beat began testing synthetic lubrication products. Without a doubt, the best synthetic oils do perform well in extreme heat (over 300 F) and extreme cold (below 32 F), but by the nature of Mazda's rotary engine, the oil temperature never exceeds 250 F without severe engine damage due to other factors. In Souther California, we have difficulty seeing the low-temperature benefits: however, when we put synthetic lubricants in the engine, transmission, and differential in our IMSA GTU race car, we immediately saw what we later found to be a common result: The oil temperature in all three locations dropped 5 to 10 F for the same operating conditions. This is apparently due to two factors: reduced friction between sliding surfaces, and reduced foaming. As we continued to use synthetic oil products it became clear that they genuinely reduced wear. We also found benefits in street use. On two occassions, cars with "scratchy" transmissions synchronizers were completely cured by a change to synthetic gear lube. After many years of experience with these products we have observed only one problem: because of the reduced friction, the time necessary to break in an engine, transmission, or limted slip differential (standard differentials are no problem) is excessively long, so we recommend using mineral oil in all three for a time to ensure rapid break in."