Just go to Wal Mart and get 3 qts 75w140 synthetic Wal Mart brand gear oil. It is the least expensive and will work just fine for you. No need to spend 2x as much for Mobil 1 or Royal Purple. I used
that I got on Amazon (I had a gift card, but even full price it is just a bit more than the walmart brand).
You can also pick up a tube of gasket maker while there to reseal it (I used Ultra Black on mine and no problems).