Mark -
While I am certain that Combo-John will have something to add to this, here are my thoughts:
First, you have 90-weight oil in the transmission? Whirlpool standard oil was 60-weight. In the automotive world, engines experience something called "pumping losses" when heavy oil is used, which robs horsepower, efficiency, and performance. I would not have put something so heavy in there as it could be dragging the entire system, but to what degree I can only speculate. Use only 15 ounces. Too much may be a hindrance as well. This is probably only contributing to your problem; I would be surprised if it explains all of it.
If not using spec. Whirlpool FSP gearcase oil, I would use 50 or 60 weight automotive racing oil (light on the detergent). Last I checked, Valvoline had these available at Auto Zone.
The only thing I can think of when you say "toothed bushing that rises up to meet the spin tube" is the set-screw that marries the clutch and pads to the spin tube. This set-screw can cause a major decline in spin performance if it is not engaged properly with the spin tube. In fact, it was almost the death of our 1974 Kenmore when the set screw backed out, causing the clutch to slip on the spin tube. Spin performance did exactly what you describe. First the machine would not spin at full speed. When it wore further, it would not accelerate unless given a hand assist, then not at all.
Check to be sure the set-screw is flush with the outer rim of the brake drum. If it is not, it is not fully engaged. The best way to tell is to look down into the spin tube and see the set-screw peeking out of the hole from the inside. I have found the best way to ensure proper installation of the set-screw is to compress the brake with C-clamps or a vice attached between the brake drum and the lower rim of the spin tube.
Another possibility is a mis-aligned spin-tube in the bearings. This can be installed in a bind, and spin performance will suffer. In these cases, obvious drag on the belt can be seen, which slows the belt sometimes to a crawl. To relieve a bearing bind, take the tension off the belt and lower the three trans bolts at least a half-inch or so. Remove the basket and agitator so nothing is putting weight on the agitator shaft or spin tube, and giggle the transmission and/or baseplate side to side and push up and down on the transmission. I try to do this with the machine sitting upright. It's a bit awkward, but I think it works best vs. the machine being on it's side.
When re-tightening the bolts, do so using a gradual star pattern like you're tightening a car rim. Do not tighten any of them all at once, as this will also create a bind.
Also, examine the basket drive. It shouldn't have oil on the drive surface of the pulley, or on the clutch pads. If they are dry, you don't have a slipping problem due to oil. If they are not dry, you should try to ascertain why.
As I said, there may be more to comment on here, but this is where I would start if given your situation.
Out of curiosity, what does the belt-speed do when the machine is supposed to be / or is trying to spin?
Gordon
