does anybody use the synthetic oil, Amsoil? this product extends the oil change interval 12months or 25,000 miles , which ever comes first if you use their oil filter too. The cost is less than more frequent oil changes , but Mazda is quick to include the phrase , voids the warrenty on almost anything that they do not sell. I can just hear them now, you went 12 months without changing the oil? What they don't know won't hurt them i guess.