Id stick with the monograde unless youve had your box recently rebuilt to OE tolerance, and even then personally I would.
In any event its difficult to find 20/50 oil without acid additives, I think only Castrol & Comma still do a classic recipe, and acid additives are definitely bad for the bearings in the O/D, shortening their life considerably.
Cheers
Jules
RatSport www.ratsport.co.uk