Pennzoil Synchromesh or DexMerc III. Pull bottom plug, drain, re-install plug and DO NOT GORILLA OVERTIGHTEN IT OR YOU WILL CRACK THE CASE. Then fill to spec through the shifter, its the easiest way to put fluid in these things. Whenever I hear of someone buying one of those bottle pumps and filling through the fill hole I laugh. I believe the correct spec for a T-5 is 3.25 qts but I'm not totally 100% on that anymore as I have not had one for a good 10 years now so make sure you look it up.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.