the reason for using atf in the carb dashpots is that the "viscosity" remains the same no matter what the temp is under the bonnet, so giving a more consistant amount of resistance to lift, unlike oil.
hope this info is of some help
I found it difficult to understand why an ATF wouldn't change viscosity with change of temperature so did a bit of browsing...
From Mobil's data sheet for their Delvac Synthetic ATF:
Viscosity, ASTM D 445
cSt @ 40º C 39
cSt @ 100º C 7.3
From Redline's data sheet for their Racing ATF:
Vis @ 100°C, cSt 10
Vis @ 40°C, cSt 53.7
I've never had a problem using engine oil in my 55 years experience with SU carburettors, whether it be the straight 30 that they specified before the days of multigrade oil or the 20W/50 specified in later years. I experimented with alternatives back in the days when I wanted to get faster acceleration from whatever car I had at the time, but none seemed to work as well as the correct engine oil.