Originally Posted by
Dubcee
8000km on synthetic is recommended in the service manual and is just fine on quality oil.
Every 4 months is a little excessive but the reason is, if the car is always sitting/frequently driven short distances and can't get to temperature, moisture collects in the oil. This moisture is usually burnt off while the engine is at proper temperature, but accumulates if it can't get a chance. This reduces the effectiveness of the oil and will eventually make a change necessary.
Regardless, if you don't follow the schedule, Mazda could deny warranty claims for defects that could be caused by poor oil quality.