The Dana 30 front takes 1.25 quarts of 80W-90 GL-5. The AMC 20 rear takes 2.4 quarts of the same. Both have a fill plug and a drain plug — no cover to drop unless you want to inspect the gears or run a full clean.
This is the maintenance item that gets skipped longest because nothing visible signals that the fluid is dead. By the time the diff is whining, the gear faces are already pitted. The standard service interval is every 30,000 miles for daily-driver use, every 12,000 miles if you wheel hard, and immediately after any water crossing deep enough to submerge the diff cover. Water in gear oil turns it the color of coffee with cream — if you see that on the dipstick or fill-plug threads, change it now.
Both axles use the same 80W-90 GL-5 gear oil for open-diff applications. If your CJ-7 has the Trac-Lok limited-slip option in the rear, you also need a friction modifier — without it, the clutch packs will chatter on tight turns. Mopar friction modifier is the factory spec; one bottle treats one diff. Synthetic 80W-90 (Valvoline SynPower, Mobil 1 75W-90) is a worthwhile upgrade — better cold-weather pumping and slower oxidation.
Drain and fill plugs on the Dana 30 are both 3/8 in. square drive — you put the ratchet end directly into the plug, no socket needed. The fill plug is on the front face of the diff (between the pinion and the cover); the drain plug is below it. Old plugs can be tight. If you can't break the fill plug loose first, don't drain — you'll be stranded with an empty axle and no way to refill.
The AMC 20 rear uses the same 3/8 in. square drive on the fill plug. It's the differential many owners love to upgrade — the two-piece rear axle shaft is the well-known weak point, but the ring gear and diff itself are stout. Fluid change is the same procedure as the Dana 30, more volume.
**The "loosen the fill plug first" rule is absolute.** Crack the fill plug before the drain plug, every time. Same logic as oil pan plug + filter — if the fill won't open, the drain stays closed.
| Part | Vendor | Est. price |
|---|---|---|
| Valvoline SynPower 80W-90 GL-5 gear oil (1 qt) — open diff | Valvoline | ~$12 |
| Royal Purple Max-Gear 75W-90 (1 qt) — limited-slip safe | Royal Purple | ~$18 |
| Mopar friction modifier (for Trac-Lok limited slip) | Mopar | ~$14 |
| Permatex Ultra Black RTV (if pulling the cover) | Permatex | ~$10 |
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.