The manual says that i should never use synthetic oil....can i use it? wats better?

 

Search.

 

since a rotary burns oil in its natural process using synthetic is bad because it doesnt burn the synthetic oil completely leaving build up which is bad over time...

 

That is a blanket statement and incorrect and in error. You need to do some investigating before you say/post things like that.

Every synthetic made burns at or below 500F. Well below the actual combustion temps of a rotary engine. In addition most if the better quality synthetics (redline/amsoil/royal Purple/Neo/mobil1) all burn with less ash (what is left after burning) than the vast majority of conventional oils.

As posted many times before, it is the poor quality semi-petrol based synthetics (like Castrol Syntec, Valvoline, Havoline, etc) that you should stay away from in even a piston engine because of a higher ash content (but even so is better than many conventional oils).

Again the only reason that the owners manuals say not to use synthetics, is because at the time of publication there were no decent synthetics, and Mazda could really say "don't use Valvoline Synthetic, but its okay to use Royal Purple".

But homey_v you should have searched as this subject has been covered many many times already.