Engine oil is fine but it’s a bit smelly. Castrol R 40 is what you want if you want it to smell like it lives on a vintage race circuit like Goodwood, Brooklands etc.
Any modern bike oil is also fine and a lot less smelly!
1950s chain?
If it's been good for 70 years then your choice of oil is unlikely to do much harm.
I use a Castrol ATF applied with a (suitably vintage) Valvespout oiler on my 1952 Phillips - having said that, the chain is relatively new, replaced when I restored it some years ago.
What chain oil would anyone recommend for a 50s bike?
Is there a modern equivalent of what would have been used at the time which would be best suited for the
components?
Or is it best just to use something modern.
Thanks