Well, there are a lot of twists to this that threaten to bring on another full-blown oil debate. I'm no oil expert, but I do have a few comments.
As I understand it, oil in the days when our bikes were made did not have limits on the amount of zinc included. Therefore (again as I understand it), flat tappets were well protected against wear. Today, zinc is restricted, so I use Valvoline VR1 Racing Oil, which has about as much zinc of any oil I can buy at the local auto parts store.
As has been recently explained here (so, yet again, "as I understand it"), it is the detergent in motor oil that suspends particles for filtering and not the elements that control viscosity behavior through various ambient temperatures as seen in multi-grade oils.
Finally, if BSA was so concerned about using straight (single-grade) oils, how would that explain what is seen stamped into the top of the oil can in the attached pictures? (That can is a prized possession.)
I am happy to be corrected if wrong but, then, I am going to ask those who are oil gurus to get on the same page with their stories so that us oil novitiates will not be confused.
Richard L.
Adding a close-up (with my daughter accidentally-on-purpose in the background).