change diffential oil

how do you change the differential oil in a 92 mazda b2200? also what type of gear oil should be used

First, loosen/remove the FILL plug (just so you now you'll be able). I use a 24mm or 15/16 inch six-sided socket, typically put a band of duct tape around the bolt head, as I think this might be one of those strange 23mm bolt heads.

Then loosen and remove the drain plug. Then replace, and use a pump-top filler (screws onto gear oil bottle) to fill up the differential until it starts to seep out the fill hole. Figure 2 quarts gear oil, I use 80W-90 Valvoline.

And do your manual transmission similarly, same gear oil.