I know this is an older thread, but I thought I'd add the information I recently (Oct. '15) received from Tremec when I asked them the same question. The answer was Mobil 1 Synthetic ATF. In my question to Tremec (via the tech line on their webpage) I referenced the recommendations for Dextron III and Pennzoil Synchromesh and even asked if I should break in the T56 with the Dextron and then switch to something else or stay with the Dextron and Tremec clearly stated in its reply that " the correct fluid for your Magnum transmission is Mobil 1 Synthetic ATF".
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