Lucas what? Gear lube additive?
You said gears looked good. Was the wear pattern on the ring gear teeth, proper?
Did you measure or feel the backlash between the ring gear and pinion? Have you tried to rotate the pinion flange some to feel for how free it is?
Have you looked up the correct spec for oil in your differential and if it needs a friction modifier ? Adding additives to differentials that have friction locking devices ( I do not know that you do), is not a good idea unless it is the friction modifier called for by the manufacturer.