I just replaced the hydraulic transmission oil in my TO 35.
I ordered the gl-1, 90 weight mineral oil from NAPA at about $13 per gallon.
As others have said, in the cold months the lift is slow at first. Some use std motor oil 10w30, they say, the lift will work faster in the cold weather.
It may be "an old wife's tale" but Ferguson used the mineral oil in order to protect or properly lube the brass bushings in the transmission.
Who knows for sure, lubes are much better today than they were 40-50 years ago.