how can i change the oil on the gearbox of a mazda 323?

If you must absoulty do this yourself then you should look for the drain plug on the bottom of the transmission to drain the gear oil out of the transmission. then there should be a fill plug on the side of the transmission about half way up the side of the transmission. make sure you do not over fill the transmission aftar draining the old oil out though. you should be able to feel the level of the oil by incerting your little finger into the hole and slightly bending your finger. make sure you refill the oil to that level.

Mickey-G
September 2008