Changing rear diff fluid

that additive i believe is friction modifier and ull prob need 2-3 bottles of gear oil as well ....dont quote me, i COULD be wrong... but im almost positive its what you will need...

I just did mine over the weekend. I used Lucas 80-90w gear oil. The additive is the for the clutches in the rear end, but I think if you use any synthetic gear oil, you dont necessarily need the additive. I know when I added that friction modifier in my '96 stang's rear, it ended up being a one wheel wonder from that moment on... I dont know what else would have caused it to do that...

two bottles 80w-90 thats all. NO friction modifier.

I've always stayed away from synthetic lube in trac lok diffs. Ford did it from the factory for a year or two and stopped it b/c they had too many issues. Word on the street was the stuff is just too slippery for the clutches to work effectively. Your best bet IMO is 2 Qts of motorcraft lube (75-90 or 80-90 hypoid gear lubricant) and one 4 oz bottle of Ford friction modifier (C8AZ-19B546-A). The rear technically calls for 3.75 Pts of Fluid (1Qt = 2 Pts). Make sure to get all the friction modifier in. That should keep your Traction loc happy and the gears and bearings comfy.

Ronstang94, I was typing as you posted. I am curious why you say no FM? I've always used it with no problems, am I asking for problems?

I also avoid adding the separate bottle FM when using synthetics (generally, less viscous fluids have FM in their blend; thicker fluids won't necessarily have it).

If the rear chatters (I'd be surprised if it did), it's easy to add FM later on.

If you drive the car in cold Canada winters, you might want a synthetic. It will flow into bearings much faster than conventional fluid.

Be sure you break the filler-plug loose before starting. It has a square 3/8" fitting. Wear mechanics gloves when you break the fitting loose. Guys on here are always ripping their hands open when the filler plug lets loose.

Good luck.

HISSIN50 said:

I also avoid adding the separate bottle FM when using synthetics (generally, less viscous fluids have FM in their blend; thicker fluids won't necessarily have it).

If the rear chatters (I'd be surprised if it did), it's easy to add FM later on.

If you drive the car in cold Canada winters, you might want a synthetic. It will flow into bearings much faster than conventional fluid.

Be sure you break the filler-plug loose before starting. It has a square 3/8" fitting. Wear mechanics gloves when you break the fitting loose. Guys on here are always ripping their hands open when the filler plug lets loose.

Good luck.

Click to expand...

Add me to list of those guys! I have two scabbed over knuckles from doing mine last weekend!

The Friction modifier just is not necesary. The Ford Motor Company states that it is only necesary with the initial oil fill of a new differential. I have heard of people using friction modifier and other people not. Some who have used it have experienced problems. If not using FM causes a problem It can always be added later, like Hissin said.

I have to be honest. I caused myself more harm than good when doing a diff cover swap (Old leaking -)Pa Perf,). I have to laugh at this because I didnt know my a$$ from my elbow. I did the cover swap correct then took everyones advice on the internet about filling the diff. until fluid came out. The Pa cover has a hole to fill near the top. I added way too much, 3+ bottles. Then I went to FFW in Epping, NH and competed in true street. Drove there, kicked butt made it to the 4th or 5th round before getting knocked out. Tried to drive home and thats where it happened. The rear end blew up. Its been off the streets for a few years now, sitting in the garage and I have my Sparco Torino 2 seats just waiting to be installed. This was all my fault and I don't make enough money to get it fixed at the moment.

Do yourself a favor, if you re-use your stock diff. cover stop adding fluid at around 2 quarts (or less). If you have a different cover don't overfill it like I did.
FYI the fill hole is a square plug located at the top of your axle facing the front of your car.
Jack the rear up on level ground, block the tires(!) and you'll see it at the top.

I said I laugh at myself because I made the same mistake twice. First I used Lucas diff. fluid then looked at the website "bobistheoilguy.com" and quickly found that the Royal Purple fluid would "do the job right." Both products had the friction modifier but videos on 'Bobs' site showed air bubbles in the Lucas products which turned me off on the product.

Expensive mistakes, dont screw up like I did. Not being able to drive my car sucks.

cobra1923 said:

I've always stayed away from synthetic lube in trac lok diffs. Ford did it from the factory for a year or two and stopped it b/c they had too many issues. Word on the street was the stuff is just too slippery for the clutches to work effectively. Your best bet IMO is 2 Qts of motorcraft lube (75-90 or 80-90 hypoid gear lubricant) and one 4 oz bottle of Ford friction modifier (C8AZ-19B546-A). The rear technically calls for 3.75 Pts of Fluid (1Qt = 2 Pts). Make sure to get all the friction modifier in. That should keep your Traction loc happy and the gears and bearings comfy.

Click to expand...

Thats what Ive heard as well and had a friend that had a Auburn/Eaton (cant remember which) that needed a rebuild b/c of synth oil. I just run the NAPA Gear oil in mine. So far so good.