So are you suggesting trying to find an oil blend that matches those made in the 50's? Those blends are simply not made any more.
The operator's manual for our FL120s, which were assembled in 1973, states that for our climate the correct oil to use is 30 wt. There are plenty of choices for 30 wt motor oil these days. We use Chevron Delo 400 but there are other brands as well.
What our manual doesn't recommend are multi-viscosity oils. (Later manuals for the same engine apparently do, however, if forum posts to that effect are to be believed). And of course the manual doesn't say anything about synthetics since they hadn't been developed for mass market use (or at all) in 1973.
My point is
not that running multi-vis or synthetic or synthetic blend will damage the engine. Synthetics can cause non-damaging (I assume) leak issues, but the engine isn't going to sieze up or anything.
My point is that there is no significant advantage to using multi-vis or synthetics in this 1950s engine design. Running the engine on the oil it was designed to run on--- single weight dinosaur/tree fern oil--- will provide all the lubrication the engine needs for the service life of the engine. A service life that will most likely end as a result of a failure that isn't caused by insufficient lubrication.
The first vehicle I ever bought new is a 1973 Land Rover Series III Model 88 which I still have. It has a 2.25 litre gas engine wich is the petrol version of the engine Land Rover initially designed and produced as a diesel. So it has some diesel features on it--- roller followers on the pushrods, for example. Very clever on their part--- a diesel and a petrol engine for different markets, but just one engine assembly line.
Anyway, from day one I lubed this engine with Castrol 20-50 because that's what the [1973] owners manual recommended. I changed the oil and filter every 3,000 miles without fail. When at some 130,000 miles it burned an exhaust valve, we took the opportunity to have the whole engine checked over since we had the head off anyway. The engine specialist who did the job here in the Seattle area said that the inside of the engine-- the cam surfaces, pushrod rollers, crank bearings and conrod bearings--- looked like they had never been run they were so clean and wear-free.
So I am a total believer in using what the engine manufacturer said to use, and if that's unavailable for some reason, the closest thing to it. Sure, there have been all sorts of advances in engine lubricants. It's the only way the engine designers could do what they've done and continue doing.
But straight-weight 30 is still available all over the place in the case of the FL120, and so is dino/tree fern 20-50 in the case of my 1973 Land Rover.
I could use a newer-generation lubricant in both these engines, but why? I would realize no benefit whatsoever in terms of the service I'm getting out of either one of them.
Now going the other way, putting sraight weight 30 dino/fern oil in my wife's new Subaru, would be a very bad idea indeed.