I have had a look through various operator manuals I have for different Rototiller models, see below:
50 & 30 Castrol D or Swanshot ST
50, 51 & 30 Castrol Swanshot or Shell Retinax, one copy has an erratum slip stuck in which also lists Mobiloil C
35 & 56 Castrol ST or SAE 90
Unfortunately none of the publications have any dates. The manual that came with my 1948 model 56 has the lubrication page missing.
One of the important points is to use an ST rather than EP oil. I use Morris Oils AG140.