I don't know about you all, but I read up on rear end gear oils and heard Royal Purple was good. I went with that, and it stated it has friction modifier in it already. Running stock gears. 100k miles when I changed over. Well, 20k+ miles later, my rear end whines ALL the time. Acceleration and Deceleration, in neutral, high speed, or low speed. ALL the time. So, I changed it for Mobile 1 with friction modifier. The noise hasn't gone away, maybe because damage is done, I don't know, but now my clutch packs wirr and almost sound like whales whining when I turn. I noticed it first at low speed sharp turns. Now that is real bad and does it at medium angle, medium speed turns too. Been googling for a bit and watching youtube. Found supporting evidence that the friction modifier could be to blame. Found one guy who has the EXACT same noise 1 month after he had replaced his rear end gears. He found out they didn't add friction modifier. He added it and the noise went away. I will be doing that this weekend. So as stated on my subject on this post, my experience has been bad. I DO NOT recommend Mobile 1 for clutch packs, at least not without adding your own friction modifier. As for Royal Purple, I can't rule out other things being the cause. It is possible my drive shaft carrier bearing could be going out, only because it is ALLLLLLLL the time I hear whining. So, I am not sure Royal Purple is to blame there, but Mobile 1's friction modifier is garbage so far. Any other thoughts or experiences from you all?