Greetings: I would like to change the fluid in my rear axle, but I can't seem to locate the filler plug? Is it on the axle shaft? I would just like to pump it out, and fill it back up via a simple hand pump. I have some Mobil 1 75w-90. Would 3 bottles be sufficient or would I need closer to 4 bottles. I have some extra Valvoline Dura-Blend 80w-90 which I was going to use for the front axle. How many bottles does the front axle take? Thanks in advance! I am looking to change the fluid more often say every 20,000 or so miles?