You dont say whether your engine is recently rebuilt (and therefore "clean").
I have not long ago rebuilt a Swift 1100cc 4 cyl side valve 1924 and is similar to your engine, and ran miller running in oil,then Castrol GP50 as I had a thermosiphon cooling (no pump) and wanted to keep oil pressure good under high temperature. A friend who also has a Swift went with penrite shelsley heavy 40-70 and reported no problems and has done several 1000's miles.
If the engine is old and has a lot of potential sludge./carbon I would stick with monograde non detergent(ie SAE30/40 or 50). Depending on your cooling characteristics/state of your oil pump and system pressure leakage use 30 or 40 or 50.
My personal preference is the Castrol GP50.
If you have rebuilt and are starting from clean you could use multigrade.
Hope that helps.
Graham