Stuart, Josiah - "hypoid" is a term used interchangeably w/ "EP" or 'extreme pressure' and has to do w/ the
cut of the gears , sort of curved for gradual meshing, so require oil additives that work against 'sliding' - 'non-hypoid' are the straight, square cut gears that mesh much more 'harshly' - it's the rear [ring/pinion] that are normally hypoid, trans gears aren't
the problem w/ hypoid rated oils in the trans is they
defeat the friction needed by the sliding syncros to work properly - so I'm told

maybe as Novak sez the additives in hypoid oild are more sulpherous as well
Bill