Both YJ transfer cases — the NP207 (1987–1988) and the NP231 (1988–1995) — use ATF, not gear oil. Drain plug bottom, fill plug side, 2.2 quarts on the NP207, 2.2 quarts on the NP231. The whole job takes about 30 minutes.
The single most common YJ transfer case mistake is dumping 80W-90 into it because it lives next to the differentials that *do* take gear oil. The NP207 and NP231 are both chain-driven, with an internal oil pump that depends on the lighter weight of automatic transmission fluid to circulate. Gear oil starves the pump, the chain wears, and you find out about it when the case starts whining in 4-high. Use ATF.
Either Dexron/Mercon ATF or ATF+4 is acceptable in the NP231 — the chain and synchros aren't fussy. ATF+4 is the modern Mopar spec; Dexron VI works fine and costs less. The earlier NP207 (only in 1987 and very early 1988) was originally spec'd for Dexron II. Use any current Dexron-compatible ATF and you're covered. Capacity for both is roughly 2.2 quarts — but always fill to the **bottom of the fill plug** rather than measuring; that's what "full" means on a transfer case.
The drain and fill plugs on the NP231 are 3/8-inch square — your 3/8-inch ratchet head fits directly into the plug with no socket. The NP207 uses the same square pattern. Crack the **fill plug first** before draining; if you can't get the fill plug out, you can't refill the case, and you do not want that discovery after the case is empty. Once the fill plug breaks loose, remove the drain plug and let it drain into a pan. Dark, smelly fluid is normal. Metal flakes on the magnetic drain plug are normal. Big chunks of metal are not — that's a sign of internal wear and a case rebuild is on the horizon.
Refill until ATF dribbles out of the fill hole, reinstall both plugs with fresh crush washers, torque to **25 ft-lbs**, and drive it. Cycle through 2H, 4H, and 4L on a quiet street and listen — a healthy case is nearly silent. Whining, clunking, or a hard 4L engagement points at chain stretch or worn synchros.
| Part | Vendor | Est. price |
|---|---|---|
| Valvoline DEX/MERC ATF, 1 qt | Valvoline | ~$8 |
| Mopar ATF+4, 1 qt | Mopar | ~$14 |
| Drain/fill plug crush washer | Dorman | ~$3 |
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.