I would check out Ken Cook Co. for an OEM manual. They will either have an original, or they can produce a photostat. I have used them for several.
Regarding the oil mix as stated in the Zephyr Owner's Manual, remember that they were assuming the use of straight 30-weight oil. Still with a bronze bushed motor such as yours, oil film thickness is important. You could probably work just fine with modern TCW-III oil in a 24:1 ratio but no less. It would still be advisable to run 16:1 even if you are running synthetic oil just because you will have adequate oil film thickness between the bearings & journals.
TCW-3 oil has a lower viscosity than SAE 30, so the 16:1 is very important. Yes it is a much better oil, however, as mentioned the oil is needed to prevent metal to metal contact, especially in the connecting rods
