On the back plate of the diff, there will be two bolts. The lower one (bottom) is to drain it, and the one a little higher (nearer the middle) is to fill the sucker back up. Once liquid starts pouring out of the upper hole, then she is full.

If you have changed your tranny fluid, this is the same thing.

The Simple Green is magical, just a few squirts and simple wipe off worked wonders on the inner hatch area and under the hood, though more to be done. Promised pictures, here are a few, although only spent a few minutes. I'm sure that actually using a little muscle and more time will remove all the rest of the buildup. Amazing, love it! Under the hood found a disconnected wire or cut from the battery, so more to explore. Tough to find schematics/diagrams/pictures of the 1971 240Z details under the hood, but will keep truckin' to find. Thanks again.

http://asta.bizhosting.com/240Z-Questions.htm