Differential lube change

Does anyone know of any reason that I should not change the fluid in the differential of my Star Chief?
I read in the shop manual that the reason there is no drain in it is that they don't want you to change the fluid in it. (of course they may not have thought that it would last 55 years) Its pretty black.

Its not a posi so I would assume regular 90 weight gear oil for it. ?

Thanks

How about you Catbird, any opinion on this one?

wydtrac.....I would venture a guess, that 99% of the rear wheel drive cars of that era, went to their grave with the gear lube installed from the factory still in them. If you're a belt and suspenders person like me, suck the old oil out and put in fresh non-synthetic oil made for differentials in, 90 weight is usually whats used......John

Thanks catbird. That's pretty much what I figured but feel better with your confirmation.