For our old engines I stand by the notion that any modern engine oil will work not only fine but much better than the best oil of the day when they came out. Not like these things are doing 10K Rpm and 2 Hp/ cui . More like 1000 rpm and 4 Cui/ hp! :0)
As said, keep it clean and changed regularly and you'll be fine with any new oil.
I did read somewhere that there are additives in oil these days (detergents?) which are designed to keep the shite that builds up in an engine in suspension in the oil; these oils being designed for full-filtration systems where ALL oil going through the engine first passes through the filter.
Most Listers & 'oids don't have oil filters, so it's "preferable" to use a non-detergent (? I might have the wrong additive here...) oil so the particles tend to sink to the bottom & form sludge, rather than being constantly thrown around the engine by the oil system.
Of course, I read that on the internet, so it's quite possible it's utter bollocks... I'm basically with glort on this one - own-brand supermarket oil is still going to be WAY better than anything that was around in 1930 when the CS was designed (and bear in mind the crankcase & oiling system predates that, being seen on the "A" which was around in the early 1900s), so I rather suspect you can't go wrong with it.