My rear diff is leaking at the pinion seal (wasn't before jacking up the motor with a lift bar and dropping the k-member to do the oil pan, not sure how, but it started that night after lifting the motor). Going to have to replace the seal, and replace the gear oil and all that. I'm looking at either Royal Purple Max Gear or Mobil 1 synthetic. The kits you buy from Ford Racing come with the Royal Purple and Ford friction modifiier, but the gear oil actually has friction modifier in it (at least the Royal Purple does, the Mobil 1 doesn't say so I'm assuming not). My question is, do I need the friction modifier if I get the Royal Purple, or not? If i need it with the Royal (which is 21 bucks a qt), would I be ok with the Mobile and a bottle of friction modifier (since the Moblie is half the cost)? Wish I had the cash to put towards just replacing the stock gears with 3.79's, but just can't do it at the moment... Just another thing to add to the ever-growing list of things I need to do to my car, lol.