I have the original t-98 in my 59 f100 and have had it rebuilt by a professional shop. It works wonderful however I have had a small ( few drops ) leak for a couple of months now. I have several questions.
1. how do I refill this trans?
2. what is the absolute best / highest quality fluid I can use? Amsoil?
3. since my leak is so small can I use a leak fix additive and refill? 

Should be a 3/4" pipe plug on the side of it somewhere. Not familiar with this particular model. GL4 spec 90 W gear oil or something close like 85-90 weight. Specific brand is not as important as using GL4 rather than GL5 or GL4-5. GL5 can damage brass synchronizer rings. If I was not sure what was in it I would drain and refill it. If it has synthetic oil in it that could possibly be causing the leak. It sometimes may leak where conventional oil doesn't. Never personally had any luck with additives.  

Thanks slumlord. So do I fill it until it runso out from the pipe plug? Drain plug should be obvious and on the bottom? Thanks

Yes, until it starts to run out. Not critical as long as its close to the top of the fill hole. A lot of old things that hold oil  tend to sometimes leak a bit. A little makes a lot of mess. Just check the level occasionally 56Roger also has good advice..