Just to be sure I went to the AutoZone site as they have lots of teck stuff on line & this is what the 4-speed selection provided
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Transmission Fluid Type
For your 1973 Dodge Challenger:
SPEC-FLUID-TYPE/WT AF Dexron III
Optional: GL-4 or GL-5 gear oil 75W, 75W-80, 80W-90, or 85W-90 may be used for warm weather operation
Lubrication Hotline
Call 1-800-373-LUBE (5823) for technical information on engine oil, gear oil, grease & transmission fluid. Open monday through friday 8AM to 5PM central time.