OLA

no way would the amount of primary oil loss OP is talking about ( "Every 60 to 100 miles I have to add 100 to 150 ml of oil in my A65T 1970 primary case,") be accounted for by a bit of oil mist from primary being drawn into the sump in such a short time ,

as for the suggestion of "but you can try to turn the seal the other way round next time you are in there." ........why on earth would anyone do that when they can get a new seal for $5/10 maybee

BSA got a few details a bit wrong but i think they designed the seal facing the right way