Found this on the Tremec Site
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From their T5 Service Manual........
2-4. APPROVED LUBRICANT. Most T5
transmission models use DexronÒ II automatic
transmission fluid. Refer the vehicle owner’s manual
or service manual for lubricant specifications.
CAUTION
Do not mix different bands or types of transmission
lubricant. DO NOT USE GEAR
OIL IN THE T5
TRANSMISSION SINCE THIS MAY DAMAGE THE
BLOCKING RING MATERIAL.