If your AT is like the one in my 99 Trooper you have two downwards facing plugs. The higher one is the fill and the lower the drain. AFAIK the level is set correctly when fluid just drips out the upper hole with the engine running (AT in park) on a warm AT that has recently been shifted through the gears. At least that is the short version. Do a search for all the details.

Again assuming your AT is the same as mine, the drain and fill plugs have a special washer with a rubber insert. That rubber insert can tend to fail if reused. Could be the mechanic that did the original service reused the washer hence the leak. In any event if you refill the AT make sure you go back with new washers.

As for making a AT last "forever", letting any leak go is certainly not the way to do that no matter what fluid you put in it.