I will agree that thicker oil will increase the acceleration response mixture. That's the purpose of the damper system in both ZS and SU. But the point is that with a correct jet orifice and needle no oil thicker than ATF should be needed. And fuel mileage will suffer, and these engines aren't noted for economy in the first place. Piston return is not affected so much by the oil as the brass piston on the damper rod opens up in the opposite direction (sort of like a shock absorber on compression) Piston return speed does not affect idle return except for mixture strength. Your assumption that it is the advance wieghts being lazy is probably right.
My point is that thick oil and the stiffer springs (to a lesser degree in all my experience) is simply treating a symptom and not correcting the cause of this lean bog.