50 years ago Ford had a different gear oil for 'high performance' vehicles but with today's improved specs I don't think it makes much difference. Any good GL-5 gear oil would work. If its a limited slip you might, or might not, need an additive. If the limited slip groans when driven then it needs the additive which Ford still sells. As I recall the original Ford oil was an 80W but few gear oils today are single weight. Any GL-5 similar to 80W-90, 75W-90, 75W-140, etc., should be fine.
The GL number is the load rating. They did away with GL-6 which was supposed to be the oil for the heaviest gear loading making GL-5 the best rating you will find on the shelf today. Lower numbers like GL-1 are for more lightly loaded gears and not something you want to use in a differential. If you can even find GL-1 it will likely be called manual transmission oil since manuals don't really need a gear oil. Many manuals today use ATF so that they shift better.
The GL ratings are an API (American Petroleum Institute) thing but I am pretty sure the same standards are used in OZ.