add or change fluid in transmission

I have an 89 ranger 2.9L 4wd 5speed. I Think gear oil is low. It has over 100,000 miles on it and think changing the fluid wouldent hurt. first what type of fluid should I use (i think 80w is recomended but have heard of ATF in manual transmissions being used).
How do I know if fluid is low? (that goes for the differentials and transfer case too.)
I've changed fluid in automatics, dropping the pan inserting a filter/ gasket set. How do i go about this with a manual tranny, Transfer case, or the differential?

Thanks for any advice anyone has to offer.

Im not sure about what kind of fluid to use but there should be a drain plug and fill plug on the side of ur tranny. I need to change to fluid in mine but the fill plug is stripped. guess ill have to go buy a heavy pipe wrench.

For the transmission oil, it depends on if you have a mazda or mitsubishi transmission. Mazda= Mercon 2 4/5 quarts, Mitsubishi= 80W Gear Oil 2 2/5 quarts. Normally there the type will be market on the side of the transmission.

Marbles is right about a drain plug. There should be two plugs, the lower to drain the upper to fill.


Rear differential - 90W 2 1/2 quart capacity if you have limited slip be sure to add 4 oz of a posi additive