I don't believe 66 ever had a slime yellow Mack, unless one of the rigs became a spare and 66 used it temporarily.
Below is a list of the 10 '81 slime yellow Macks:
Engine 10 (later reassigned to 4)
Engine 41
Engine 42
Engine 45
Engine 46
Engine 58
Engine 85 (later reassigned to 263 then to BOT)
Engine 94
Engine 236 (later reassigned to 73)
Engine 277 (later reassigned to Dept. of Corrections)
Engine 65 had the original slime yellow rig, a 1980 ALF.