Thank you all for the comments, folks.
Another question, if I use the Redline full synthetic stuff, do I still need to add friction modifier? What harm will it do, if it does not need, and I add it anyway? Thanks. Red Line 57104 75W-140 NS Gear Oil - 1 Quart : Amazon.com : Automotive
2014 Mustang V6 3.7L 6-spd manual, with MGW shifter, Airaid CAI, BBK shorty headers, Corsa Sport axle back, strut tower brace, JLT oil separator, GT500 differential cover, BG Synchro Shift II gear oil, Redline differential oil.