Stiff, notchy shifting

I haven't been satisfied with how the SRT shifts since I bought it; stiff, slow, and very hard to downshift. Double-clutching is of little help. It shifts like a tractor, or a diesel minivan, which is the transmission it shares.

I'm currently working through a laundry list to see if I can improve it:

• Mopar Performance short-shifter: actually made it worse, but it's improving over time.
  
• Clutch pedal and crossover cable adjustments: some improvement.
  

Tonight, with Chris' help, I changed out the old 10w-30 Mobil 1 High Mileage the previous owner had in there. Now it's rocking Amsoil ATF, and seems to shift better, at least in the quick run around the block. It needs some more driving before I can really tell. I also ordered a custom Maddog shifter from Chuck, with a raised knob height and a rod brake.

We'll see...

UPDATE: After driving it to work, the Amsoil ATF seems to be a better choice for tranny fluid. It shifts much better.