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Limited slip friction modifier is the cheapest part in an axle and causes more confusion than anything else in it. Too little and a clutch-type limited slip chatters and bangs in every parking lot turn. Too much and the clutches slip so freely the axle acts open, and you just paid for a limited slip you no longer have. Some axles need it, some must not have it, and some gear oils already contain it. Here is how to get it right.
What Friction Modifier Actually Does
A clutch-type limited slip has stacks of friction discs and steel plates behind each side gear, preloaded by springs. In a turn, the outside wheel has to turn faster, so those clutches must slip. Plain gear oil gives the plates a friction curve where static friction is much higher than sliding friction. The plates stick, break loose, stick again. That stick-slip is the chatter and the shudder you feel through the seat in tight turns.
Friction modifier changes the friction curve so the plates slip smoothly instead of grabbing. It does not make the clutches weaker in a straight line, it makes the transition from holding to slipping smooth. Dose it right and you get a quiet axle that still pulls both wheels on a slick launch.
Which Axles Need It
| Differential type | Friction modifier? |
|---|---|
| Clutch-type limited slip (Ford Traction-Lok, Dana Trac-Lok and Power-Lok, older GM Eaton posi, Auburn cone type, most aftermarket clutch posi units) | Yes, unless the gear oil already includes it |
| Open differential | No. Harmless in small amounts but pointless |
| Gear-type (Eaton Truetrac, Torsen, Quaife, Wavetrac) | No. No clutches to treat |
| Automatic locker (Detroit Locker, ratcheting lockers) | No |
| GM G80 locker | No. GM specifies plain gear oil; additive can make the governor and clutch pack slip |
| Selectable lockers (air or electric) | No, unless it is a clutch-assisted design that the maker says needs it |
Not sure what is in there? Look for a limited slip tag near the fill plug or on a cover bolt, check the axle code (our axle code guide covers GM G80 and Ford L-tags), or raise one wheel with the transmission in neutral and turn it. If the other wheel turns the opposite way, it is open. If it turns the same way or resists, it is a limited slip or locker.
How Much to Use
The standard answer for most clutch-type units is 4 ounces per axle on a full drain and fill. That is the factory call on most Dana and Chrysler Trac-Lok axles and the most common figure on Ford 8.8 and 9.75 Traction-Lok service information, though some Ford applications call for more. If your service manual or the gear oil maker gives a different number, follow it.
| Situation | Starting dose |
|---|---|
| Conventional GL-5 gear oil, clutch-type limited slip | 4 oz |
| Synthetic gear oil labeled limited-slip ready | 0 to 2 oz, add only if it chatters |
| Fresh clutch pack rebuild | 4 oz plus soak new frictions in oil and modifier before install |
| Old, worn clutches that chatter | 4 oz, then 2 more if needed. If still chatters, clutches are glazed |
Never go past roughly 8 ounces total in a normal passenger or light truck axle. Past that point you are not quieting the clutches anymore, you are greasing them. The axle will be silent and nearly open.
Limited-Slip Ready Gear Oil
A lot of synthetic gear oils are labeled for limited slip use and already contain friction modifier. Adding a full 4 oz on top can overdose a sensitive unit. The rule we use in the shop: if the bottle says limited slip ready or contains LS additive, fill with it, drive the figure-eights described below, and only add modifier if it still chatters after 50 to 100 miles. Add in 2 oz steps.
Do not mix brands of modifier and LS-ready oil and assume they stack predictably. They do not always.
Getting It Into the Clutches
- Drain completely. Most axles have no drain plug. Pull the cover rather than sucking oil out of the fill hole, which leaves old oil in the bottom of the housing. Clean the magnet.
- Fill the gear oil first, modifier last, or pour the modifier in with the first quart. Either way, fill to the bottom of the fill hole. A quart-bottle fluid transfer pump ($19.99) makes this clean and fast.
- Drive 10 to 20 slow figure-eights at full lock in both directions in an empty lot. In a 4x4, stay in 2WD. This forces the clutches to slip and works the modifier between the plates.
- Give it a few hundred miles. Glazed plates sometimes take a while to settle.
Signs You Got It Wrong
Too little
Chatter, shudder or a ratcheting groan in slow, tight turns, worst when cold. Occasional single bang mid-turn. Our Ford 8.8 Traction-Lok chatter fix and Dana Trac-Lok chatter fix pages go deeper on those two axles.
Too much
One-wheel peel on a wet or gravel launch where it used to pull both. Test it: one wheel up, transmission in neutral, put a torque wrench on a lug nut. If it turns with very little effort, the clutches are either worn or over-modified. Drain and refill to the right dose before blaming the clutches.
Modifier will not stop the chatter
If a correct dose and figure-eights do not fix it after a few hundred miles, the clutches are glazed or worn. Rebuild them or replace the carrier. For a street truck tired of the whole routine, a gear-type unit like the Truetrac for Ford 8.8 ($803.99) or the Truetrac for Ram 9.25 ($729.99) never needs friction modifier again. Our Detroit Locker vs Truetrac guide covers the choice.
Do Not Put It in the Transmission
Limited slip additive is for the axle only. It does not belong in an automatic transmission, a manual transmission or a transfer case unless the maker specifically calls for it. In an automatic it changes clutch friction and can cause slipping and shudder. For the right fluids there, see our fluids section, and for transmission parts, CoreTransmissionParts.com.
Quiet Axle, Real Traction
Gear oil, a pump to fill it right, or a gear-type limited slip that never needs additive again.
Shop Fluids → Shop TruetracFAQ
How much friction modifier should I add to a limited slip?
Most clutch-type limited slips take 4 oz per axle on a full drain and fill with conventional gear oil. With synthetic gear oil labeled limited-slip ready, start with 0 to 2 oz and add more only if it chatters. Follow the service manual if it gives a different amount.
Can you add too much limited slip additive?
Yes. Too much modifier lets the clutches slip so easily the differential behaves almost like an open diff. If one wheel spins on a slick launch, drain and refill with the correct dose.
Does a GM G80 locker need friction modifier?
No. GM specifies plain gear oil for the G80 automatic locking differential. Friction modifier is not needed and can affect how it engages.
Does a Truetrac need limited slip additive?
No. The Eaton Truetrac and other gear-type limited slips like Torsen have no clutch packs, so they run on plain GL-5 gear oil.