As I understand it, this issue has a lot to do with what happens in theory, and what happens in practice.
In theory, the viscosity of the dashpot oil is irrelevant to the piston's movement. A set amount of vacuum will result in a set amount of piston rise, and a set station on the carb needle. All the damping does is affects how long the piston takes to get it to its maximum height for a given vacuum.
In practice, your foot is never still on the pedal, the road surface is never 100% flat, there is always a wind, and it is rare for these parameters to remain constant for long enough to allow the piston to reach its optimum height before everything changes again. At this point, the damping comes in.
Low viscosity and the piston will move quicker. It is therefore more likely to get to the correct height before a change in vacuum, and will be correctly set for more of the time, but every tiny blip of the throttle at idle will waste excessive amounts of fuel as it jumps upwards.
Higher viscosity, and it will be leaner for longer under vacuum, benefitting economy, although pinking may occur under hard acceleration. But as the damping is double-acting, the piston will also be slower to fall, thereby staying richer for longer on the overrun, and taking time to settle to a smooth idle. So it's swings and roundabouts with this approach.
I'm personally a fan of experimenting with these things - every car and driving style is different, so find what fits!
Michael