Just some guy
Join Date: Jun 2001
Location: SC foothills, USA
Yep, GM specifies their own additive for "posi's".
You use the additive for any "clutch" style limited slip rear differential. "Locking" style rears don't need it, nor do the most common "open" rears. You might look up lockers, very few people use them in old Fords, but they are out there. Known for making rather interesting clanking noises in corners. Open rears are the ones when when you spin the tires, you actually only spin one of them.
At one time Ford sold THE best friction additive. Seriously. Even the GM and Mopar guys would slink into the Ford dealerships to buy it. That was a long time ago. Today GM and Mopar have their own additives which (much as I hate to say it) are quite interchangeable and as good as Ford's. BUT, the generic branded stuff in the parts stores is worse than useless. Do not buy it. Buy from whichever one of the Big Three has a friendly parts counter nearest to you. I recommend going to Ford just so a stray shiny new Hellcat or Corvette won't tempt you away from the fold though.
Oh, I almost forgot. There exists gear oil which is labeled as having the additive included. I suspect it's one of those deals where one drop added to every 1000 gallons is enough to keep them from getting sued for false advertising. But not enough to do us or our rear ends any good. Avoid those oils.
Last edited by GypsyR; 09-07-2019 at 12:28 AM.