Is the trans a "stock" T-5 ie: not been rebuilt or upgraded to the WC "Z"?
If upgraded you can use either ATF or 70w Red Climbing Gear
oil, if it is stock they generally reccommend ATF although some of Ford's SVO's were a bit unique in their drivetrain components.
Being from NZ are there any Ford dealers that have a competent parts/service dept. close(rather lacking close to where I live )usually they can look up the information if they have the parts number and or vechicle it came out of .
On doing a search of several Mustang forums it seems the V-8 SVO Mustang for '83 used a pretty stock T-5 trans. These transmissions used ATF...'84 and later began to see more of what would become the "WC" and were beefier for the V-8's. hope this helps