J spaulding

On a 129 it is quite difficult to get the required M14 (14mm) Allen wrench on the filler cap (left side of diff) to get it open. The drain plug (right side)
is easy. I bought a 14mm Allen wrench for $9 and cut off just less than .75 inches of the short side. This small chunk is now a very short Allen wrench and works beautifully on the filler cap with nothing more than a 14mm box wrench to open it. The original Allen wrench still works fine for other things. With this stubby 14mm little slug you can change fluid in 20 minutes. I used Pennzoil "Gear Plus" at $4.90 per quart. This is a very modern product that is so slippery it will even work in a Positraction unit. It takes 1.5 quarts. You will definitely need some kind of hand pump to get it in since there is not enough room to squirt it in. I highly recommend changing your differential fluid and it makes a noticeable difference. Don't drain the fluid out until you are sure that you can get the filler plug off or else you can end up with no way to fill it back up.