The Lucas HD Primary Chaincase Oil you have linked to is pitched as a clutch stack coolant rather than a primary chain lubricant.
So it isn’t necessarily what you are looking for in my opinion.
Stick with ATF but make sure you get type F - the more common Dex (which you may have used on your other bikes) is no good if you get it on your sintered bronze clutch plates.
While the manual specifies 20/50 engine oil i’d be inclined to leave it in the can for all pre-MK3’s (the MK3 needs a thicker oil to make the chain tensioner work properly)
As Nigel says, modern oils are packed with friction modifiers and complex additive packages none of which existed back in the day and all of which will hinder the performance of your clutch stack.
ATF is nice and thin, does a great job in lubing your primary chain and doesn’t go too sludgy.
It’s also brilliant as it’s bright red in colour, so you can easily spot a leak and know where it has come from.
If your primary oil gets contaminated from your gearbox (migration of oil up your clutch pushrod) or from engine oil (blown out drive side oil seal or leakage through the three 1/4 inch UNC bolts), you can easily spot that too.
It’s important that you make sure you use a measured 200ml rather than fill up to the level plug.
Oh and by the way, if you are buying some ATF-F you can also use it in the dommie forks you are refilling this weekend - it will work an absolute treat!
