Transmission Fluid

I’m working on a 727 now. Looks like there’s atf-4 in when I removed the pan but when I dropped the vb it looked way different. Nice and dark but not burnt probably Dex 2 or 3. I just purchased truck not running because stuck in first gear. He changed radiator so i
guessing he added something different then what was originally in. My question is do I need to flush transmission out with one type fluid or can I just add to whatever is in with it now? Approximately 5-6 quarts is what I lost after pan and vb was removed.