running 5W30 is fine for the mazda3... it's just recommended to be used because of fuel economy reasons... the engine is designed to handle 5W30... it is the recommended grade overseas... if you live in a warm climate area, then it will work just fine... when the engine is warmed up, the viscosity is just a bit thicker running 5W30 than with 5W20... if it is cold where you live then 5W20 is better for the engine... but use that oil ASAP if that has been sitting on the garage shelf for a while because oil can thicken due to the cold