Welcome Ta Paul. Not sure if you've found Panteraplace.com, but it is a wealth of information. Here is a link specifically on what you are doing.
http://www.panteraplace.com/page10.htm
The ZF takes approximately 3 ½ (qts)...I used Lucas Heavy Duty 80/90 Weight Gear Oil, (or equivalent as others have posted). Fill until it comes out the side fill plug. Per Roy Butfoy, "any 80/90 Weight gear oil works well, BUT Valvoline". Note the Napa people said Valvoline makes their Napa Brand gear oil, so I went with the Lucas…a bit more expensive but good quality. Interesting Marlin is having good luck with Vavoline gear oil. Roy Butfoy is the guru of ZF's...you will see his name everywhere when the subject of ZF's comes up.
Congrats on your purchase.
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