Slick Rick, how long have you been using MTL? Also, how cold was it when you had shifting problems? I sent an email to Redline, and they stated the only reason for using ATF is to ease shifting in cold temps. No mention of better fuel economy, but I still think this is a reason also.
Chysler had a TSB for their Manual trannys that came factory filled with ATF. "if customer complains of transmission noise, drain ATF, and refill with 80W90 gear oil."