Both differentials on a JK should see fresh gear oil every 30,000 miles on the street and after every water crossing deep enough to submerge the axle tubes.
The JK's front axle is a Dana 30 on Sport/Sahara models and a Dana 44 on Rubicons. The rear is a Dana 44 across the board (Dana 35 on very early Sport trims is rare). Fluid changes are the cheapest insurance you can buy for an open or locked diff — and they're nearly identical front to rear. The factory schedule says "inspect at 30k," but on a wheeled rig that sees mud, water, or dust, plan to drain and refill at 30k regardless.
Capacities to know: front Dana 30 holds about 1.05 quarts, front Dana 44 holds 1.35 quarts, rear Dana 44 holds 2.375 quarts. Use 75W-90 GL-5 synthetic for most use cases. Step up to 75W-140 if you tow heavy, run 35"+ tires, or live somewhere hot like Phoenix. Rubicons with factory Trac-Lok rear diffs require limited-slip additive — Mopar 04318060AC, one bottle per fill. Electronic lockers on the Rubicon do not need additive.
Both diffs have a fill plug at the 9–10 o'clock position on the cover and a drain plug at the bottom. Open the fill plug first — if it won't budge with the diff full, you're stuck with no way to refill. Drain into a pan, inspect the magnet on the drain plug or inside the cover. A light gray paste is normal. Glittery metal flakes or chunks mean stop and investigate before you put fresh oil back in. Some owners pull the cover entirely on the first service to clean the housing and inspect the ring and pinion — worth doing once if you bought the rig used.
Reinstall the drain plug, then pump fresh oil through the fill hole until it dribbles out. That's the level — don't try to "top it up" past that, the breather will push the excess out and make a mess. Torque both plugs to 25 ft-lbs. If you ran the cover off, scrape both surfaces clean, lay a 3/16" bead of Ultra Black RTV, let it skin for 10 minutes, then bolt the cover on at 30 ft-lbs in a crisscross pattern. Let it cure four hours before refilling.
| Part | Vendor | Est. price |
|---|---|---|
| Valvoline SynPower 75W-90 GL-5 (quart) | AutoZone/RockAuto | ~$13 |
| Mopar 75W-140 Synthetic Gear Oil | Mopar | ~$18 |
| Mopar Limited Slip Additive (Rubicon Trac-Lok) | Mopar | ~$12 |
| Differential cover gasket (RTV alternative) | Permatex Ultra Black | ~$9 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.