I think that's a question which can only be answered by someone with extensive experience in both oils and rear axle/LSD engineering. We all take chances sometimes with oils, because of this very thing, the slight variations in viscosity. In my MG period, there was an oils specialist on the forum, who I met a few times. He worked in consultancy and the sort of guy you listen to. I never discussed diff oils because MGB's are so simple, and apart from avoiding an oil which dissolves the bronze thrust washers, it was a matter of keeping it simple, and old fashioned. We did however discuss engine oils in respect of my Roadster, and the bewildering choice of 0/30; 0/40; 5/30; 5/40 fully synthetics. The truth is, these are marginal differences and in an engine, with regular changes, you won't see much difference. Maybe a minor variation in oil pressure. These oils are manufactured to create a "solid" barrier between surfaces under extreme heat, and so he was quite laid back about the variations of those particular grades (subject to the climate you live in!).
No individual will have any worthwhile long term experience of variations in diff oil, and so in the end you take a risk whenever you vary from recommended viscosity. MMC used Morris Oils for the Trad range. Morris have been around a long time. So that's what I use in my diff and Mx5 Gearbox. The Getrag recommended oil is different. I tend to use Shell or Mobil in the engine.
