True, there is no service schedule for the gearbox like the engine-one year or so many miles deal, but that gearbox has most likely had debris ground into the oil for the last 40+ years, can't do the gears, bearings, syncros etc. any good. I say get a fluid extractor and change the oil. Try to find some 80/90 GL-4 spec, the same lube that was put in the diff housing and transmissions back when these cars were made. GL-4 has been replace by GL-5 but the Muncie wants GL-4 spec 80/90 oil. The GL-5 has sulfur additive and can harm the Munci's syncros. Phillips 66, Valvoline, Chevron etc, and others still supply GL-4 for older transmission brands. I put GL-4 in my M20 along with a bottle of GM synchromesh additive when I changed mine. You should have seen the crap that came out with the oil. I avoid synthetics.
Just my opinion.