A833 Can't Downshift / Poor Performance

Av you know, the synchronizers work by the brass grabbing the shiny smooth brake surfaces of the brake-cones.
They gotta bite.
In my experience;
Full synthetic oil is TOO SLIPPERY and the brass just slips and slips and slips. But you tried three different fluids and all same.
So next up is input shaft not stopping. So push the pedal down and have someone look. If it stops, and the car is stopped, and you still cannot engage a gear, this has nothing to do with the synchros.
But if it slips into any gear as the input gear comes to a stop, perfect, that is working good. First will occasionally be a lil harder to get if the input comes to a complete stop, because the the clutch teeth could be butted up. Simply selecting second and then first may be enough to unlock them, but sometimes you gotta spin the input just a hair.

More coming, supper time!