The exact brand I used (Midco) was sent to me by John @ HPA (sponsor) I think that Midco is the parent company of Midwest Converters.

I have seen other brands such as Gunk Synthetic Highly Friction Modified Auto Transmission Supplement (P/N M1410HFM) at stores such as PepBoys or Advanced Auto.

States:
"A uniquely blended synthetic product designed to convert & enhance DextronIII fluid into a highly friction modified ATF such as Chrysler-ATF+3 or +4. Stops torque converter shudder instantly. Extends the life of the fluid. Keeps valves & govenors free. Softens hard shifting. Inhibits overheating & fluid oxidation. Protects & coats gears & pumps. Compatible with electronics, frictions, & seals