Earlier thread on oils for Huntmaster:viewtopic.php?f=21&t=1850
Issue about detergent in oil ain't quite the same as the oil grades (you can get detergent and non-detergent in straight grades).
Plenty of oil opinions on the A7/A10 forum if you really want to go round in circles....http://www.a7a10.net/forum
I think a fair summary is that as many A10 owners happily use 20-50 as don't and similarly as many use an extra filter as don't.
Personally I'm not keen on Morris oils as with a few exceptions (Motorsport 15-50, V-twin 20-50) they don't have anywhere near enough the amount of ZDDP (zinc additive) in them that I like to see for engines that can wear cam/timing gear relatively quickly (eg Ariel singles and Ariel/BSA 650 twins). (Other folks on the forum have used Morris products for many years with no issues, I'm probably in a minority of one....)
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)