Morris Minor Owners Club

At the risk of being shot down
The heavier the oil more damping so sluggish throttle reponse.

Genuine SU/Burlen dashpot oil is a straight SAE 20 mineral oil.
Personally I've always used multigrade engine oil in my minor and previous various SU equipped cars in std form and never had an issue.

"The heavier the oil the more damping so sluggish throttle response"

Not necessarily!
I have tried all grades of oil (from 3 in 1 oil to proper SU damper oil and on through to EP90) in the SU dashpot and in some cases the heavier oil has improved the Venturi effect and the engine has picked up quicker.
https://en.wikipedia.org/wiki/Venturi_effect
It is a question of horses for courses and some heavier oils in some engines improve the performance. All you can do is try the different oils and see what happens.

When I worked in the BMC garage we just used to put engine oil 20/50 in the dash pot with out any ill effect and never had any complaints.

Shot down

Yes when I worked on bmc's in the fleet, always used std 20/50 and as I said always done same on my own cars.
Seem to remember trying 3 in 1 on a tuned 1100 allegro. Can't remember if it made a difference but probably convinced myself at the time

TBH I have tried engine oil, 3-in-1 oil and SU carb oil over the years, depending on what was at hand, and not really noticed any difference....not that I was attempting any comparisons.