GL-4 is normally recommended for older transmissions but most will agree that GL-5 works OK. There's an argument that GL-5 corrodes brass but a lot of people disagree it's an issue.
I would stay away from
synthetic. Some say it will defeat the action of the syncros. I ran
synthetic in a Muncie for about 15 years without issue, but recently changed it out to GL-4 which can be obtained at NAPA. Interestingly, the only thing I noticed different about
synthetic was that the Muncie aluminum case took on an oily sheen appearance (but dry) like the case was permeated through and through with it.
