Mick Hemmings-oil choice.

As no one appears to know I would suggest you consider a good 20/50 which will be better for cold starts.

Would high-zink content oil be worth considering/

Have always used a 20/50 mineral oil in three Commandos I've had - in the old days it was Castrol GTX while for the past decade have used Halfords Classic , changing every 1500 miles.

I had a look at data sheets for various suppliers of classic 20-50. Top of the list for specs and ZDDP content (1200 ppm) was Morris lubricants classic 20-50.  

That sounds a good bet Gordon, will get some for my 99. I would prefer a fully synthetic multigrade with an Ester base though .

... Smith & Allan at 1500ppm:

https://www.smithandallan.com/product/smith-allan-ultra-20w-50-mineral-engine-oil-3986

Cheap as well.

Why does nobody suggest this? Is there something I don't know? I noticed this in the recent posts titled "Castrol GTX 20W/50 availabilty" as well.

My understanding is that the "Classic" 20W/50 is the same product as the Castrol GTX we all used years ago, before they changed the standard GTX to the modern GTX which is synthetic based.  It has high zink and sulphur content and all the other additives that we need and none that we dont. It's no good for modern engines so that's a good sign too. 

"My understanding is that the "Classic" 20W/50 is the same product as the Castrol GTX we all used years ago"

Highly unlikely.

If my memory serves me well? If your using an oil containing a detergent it should be run through a filter.

Alan
Like you I have good memories of dropping in on Mick and Angela in Northampton. I lived in Stamford at the time so it was a short ride. I never discussed choice of oil with Mick. This was because I was then, and am now, totally happy with the oil I use in my Commando (750cc Fastback made in 1994/95 by what was then Fair Spares).
From day 1 to the present I have used nothing but Mono grade 50 with detergent, summer and winter. The brand has nearly always been Silkolene Classic Hardwick 50. On the tin (not one of those plastic containers), it describes itself as:
"Classic/ vintage SAE 50 mono-grade oil with good detergent and a high level of anti-wear additives".
I change the oil every couple of thousand miles and so far, 52,000 miles on, I have had no engine issues. I plan to be riding it 'round France at the international rally next month.
For Commandos, Norvil Motorcycles recommend Mono 50 in summer and Mono 40 in winter. A lubrications guidelines sheet I have from Andover Norton recommends 20W/ 50 Multigrade or 40W Mono grade.
Hope this helps rather than confuses.  Robin

If you are getting on a bit, a multigrade will make a 750  easier to start from cold and  get around faster on start up. An ester based fully synth oil stays on surfaces  exceptionally well  and will give enhanced protection ,albeit at extra cost and you wont find a 20/50 easy to source  ,especially on trips away. Thinner oil will get used a bit faster ,thicker oil will  run more quietly in an older motor.  I have run  my 99 on 30, 40, 50 , 20/50 and synth  Yamalube 10/40 over a number of years .  All with plenty of magnets fitted. With the thin synth oil  consumption and noise was slightly  up , but noticably a very lively performance with absolutely nothing on the magnets and a very clean motor. (no filter!). Mayonase in the tank problem dissapeared , dont know why.  Not yet tried a 20/50 or 30/60 fully synth . But would like to .  A gallon of inexpensive  Millers classic 20/50 will have to get used first.  You are spoilt for choice !.