After dumping the original fill, you can run syn for a long time in a diff. 100K miles + would be safe assuming the truck doesn't do a lot of towing, and assuming no deep water crossings (over the hub). If you can't stand to run it that long, drain it again after 50K miles. Anything less than that would be a waste of good fluid. Differentials don't get really hot relative to an engine or trans, and there isn't a lot of wear metal after initial break in. Odds are due to the age of the truck, you will need to replace an axle seal or pinion seal before the fluid actually needs to be replaced again.
Draining the factory fill early is always a good idea though since diffs have a fair amount of break in metal floating around. Usually the factory fill will be an opaque dark grey color from all the break in junk. Despite that, Ford's interval in the maintenance manual for my truck is 150K miles! I drained the factory fill at 50K. It was nasty. I then ran Valvoline DuraBlend syn blend for 30K, then drained that 30K later when I decided to go full syn. That was a waste since the DuraBlend looked practically like new. I have been running Royal Purple since and haven't changed it yet (70K miles).
Most people never change diff fluid, so you're already ahead. I suspect most differential failures are caused by fluid leaking out from a bad seal that goes unrepaired.
You probably could have just put the LS additive in the Citgo and run it for a normal interval with no issues, but honestly LS chatter would freak me out too, and I would have gone with new fluid too just because.
2002 Ford Ranger XLT | 3.0 V6 / 5-Speed Automatic / 2WD | 168K miles
1994 Ford Ranger STX | 4.0 V6 / 5-Speed Manual / 4X4 | 171K miles