Just wondering, but is it OK to replace the differential fluid with a synthetic gear lube like Mobil 1? I asked at the local parts store, and the recommendation was for the Mobil 1 75W-90 synthetic. What's the difference between Honda's product and Mobil 1?

NO! VTM-4 fluid is closer to ATF than gear lube.

To the best of my knowledge there is no acceptable substitute for the Honda VTM-4 fluid.

Read Gary Flints explanation of how the drive train in the RL works and you why you don't want to do anything other than exactly what Honda recommends!

Thanks, now I know. Great place this.