Here is an excellent video on this topic. If you are interested in only the gear oil GL information and flow characteristics start the video at 12:00. The earlier part of the video deals with a destroyed transmission repair.
https://www.youtube.com/watch?v=Z7NpsMZ9pBQ
I use this GL4 Gear Oil. When I bought it online on 5/19/2017 the price was $9.83 per gal + $14.85 for shipping + Handling Charge of $1.95 + $1.61 Tax for a grand total of $28.48.Price has more than doubled since then. Hope you find some cheaper vendor.
https://www.finditparts.com/search?u...l24238&commit=
If you have Amazon Prime shipping is FREE.
https://www.amazon.com/s?k=gl4+gear+...l_8mg5skwrim_b