SAE 50 seems to be consensus amongst pals here.
some advice from novak-adapt.com
When refilling your NP435 with gear oil, choose a GL-4 rated oil instead of GL-5. The latter has a higher sulphur content that is mildly corrosive to the bronze alloy synchros of the this and many other transmissions that do not have composite synchronizers. StaLube and RedLine are brands to look for, among others.
We recommend that you select a conventional lubricant or a para-synthetic in lieu of the full synthetics.
and look here for a rebuild w/SAE 50