Replacing M20 speedo gear seals?

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Previous owner put a Muncie M20 in my 68 but it is leaking pretty good at the speedometer gear. Going to replace the two seals there (may just replace the whole speedo "bullet" thing), but just so I know what I'm getting into- will removing and replacinng this necessitate a fluid change? Will all the fluid leak out when I remove it?

And follow up question, I've heard different things about additives for these transmissions- anything necessary or just 80-90 gear oil?
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