Harley’s factory spec for the 6-speed Cruise Drive transmission (2007+ Big Twin) is a 20W50-type fluid – Formula+ or Screamin’ Eagle SYN3. We also compared 8 heavier 75W-140 gear oils that many riders run as an aftermarket alternative, and analyzed 200+ HDForums threads on Cruise Drive longevity. Here’s the factory-correct pick plus the heavy-duty options.