Thanks to Listeroil for that useful bit of information. The Lister manual on my copy of the Utterpower CD (which I just discovered today) dates from the '50s, since it specifies compliance with US Military spec. Mil-0-2104 (no suffix).
I looked up mil-0-2104. This page
http://www.fuchs.com.sa/english/products-appendix.asp very usefully gives a running history of worldwide oil standards for both SI and CI engines back to WWII.
Mil-0-2104 turns out to be the follow-on to U.S. Army standard 2-104A and 2-104, issued between 1941 and 1943, and were superseded by U.S. Army 2-104B May 1943. 2-104B. 2-104B was the same as 2-104/104A except that it specified testing with a high-sulfur fuel. This matters because sufur reacts with water created during combustion to form sulfuric acid. Strong basic buffers must be added to the oil to prevent corrosion. The other requirements laid down in the early 2-104/mil-0-2104 standards seem to be shear resistance and extreme pressure protection. Tellingly, the API equivalent certification ti mil-0-2104A is CB. Mil-0-2104B is equivalent to API CC.
What all this seems to boil down to is that Lister expected people to change the oil often enough to get the entrained gunk out. They specified a level of detergency in later years that should keep the carbon in suspension, but would probably settle heavier particulates out to the deep sump over time.
I have corresponded with a college friend who is now heavily into antique military vehicles. There are plenty of unfiltered sumps with con-rod dippers running around in his crowd, too. The consensus there is to run a detergent oil on rebuilt engine and to change frequently. If you have vintage deposits of crud, dispersants and detergents are not your friend. If you can't be bothered to swab out your original Lister crankcase, use a non-detergent oil
Now onto viscosity: Lister does not specifiy a single multigrade oil for the CS ever. This is telling, because they were common as dirt by 1983. I suspect they didn't like what happens to viscosity improvers when the oil goes acidic. The good news is that you can get API rated single viscosity oils at your friendly agricultural supply dealer. Plenty of vintage farm equipment uses stuff that should meet API CC or CD and is SAE 20 or 30 viscosity. Likewise, the Detroit Diesel folks tend to need oils that have similar characteristics to what Lister specified.
For breaik-in of a listeroid, I recommend lots of strong magnets and frequent, hot oil changes. After break-in, change the oil when it starts to look too black or smell funny.
For Hotater: The temperatures specified are maximums. The magic minimum for just about any crancase is about 160 F. If you can keep engine oil between 170F and 220F, it lasts a very long time. Below 170F, water emulsifies into the oil, and above about 220F the oil begins to coke or oxidize. The fact that Listers and listeroids draw a vacuum on the crankcase can't but help with getting rid of the water vapor.