The 6G runs a Dana M210 front (26.2 oz, Ford spec 80W-90) and a Dana M220 rear (65.3 oz, Ford spec 75W-85 — 63.6 oz with the electronic locker). The factory fluid is pulled out within the first 5,000 miles by most informed owners because it ships dark and full of break-in metal.
Ford spec calls for the first scheduled diff service at 150,000 miles. Trail-driven 6Gs see metallic clutch and gear shed in the first few thousand miles and the differential fluid is visibly contaminated long before any service interval. Change it once at 3,000–5,000 miles, again at 30,000, then every 30,000 thereafter or after any deep water crossing.
Ford specifies Motorcraft 80W-90 for the front M210 and Motorcraft 75W-85 for the rear M220. Dana itself recommends 75W-140 in both axles — that's heavier than Ford's spec and adds thermal margin if you're towing or running 35s on highway. For a stock or mildly modified truck driven in Phoenix or anywhere hot, stay with the Ford spec; for 37s, heavy load, or sustained dragstrip use, 75W-140 in the rear is defensible. Use synthetic in both either way.
Neither axle has a drain plug. Fluid removal is either through a suction pump from the fill hole or by pulling the diff cover. The cover route is messier but lets you wipe magnet shavings, inspect ring-and-pinion wear, and start clean. Most 6G owners go cover-off the first time, then suction-pump subsequent services.
Park level, lift the truck, set stands so the axle stays level. Loosen the fill plug FIRST — the rear plug is a 3/8" square drive on the cover face. If the fill plug won't break loose, do not drain. Once the fill plug is open, either pump fluid out from above or remove the 10 cover bolts in a star pattern and let the cover drop. Wipe everything inside clean with a lint-free rag, clean the magnet, scrape old gasket / RTV off both sealing surfaces, lay a fresh bead of Permatex Ultra Black (3–4mm wide, looping inside the bolt holes), bolt the cover back at 25 ft-lbs in a star pattern, and wait 30 minutes for the RTV to skin before filling.
Refill through the fill hole until fluid runs back out — that's the level. Front M210 takes about 26 oz of 80W-90; rear M220 takes 63–65 oz of 75W-85 (the exact number depends on whether you have the Badlands/Sasquatch electronic locker). Reinstall fill plugs at 25 ft-lbs.
| Part | Vendor | Est. price |
|---|---|---|
| Motorcraft 75W-85 gear oil (rear M220) — 2 qts | Ford / Amazon | ~$32 |
| Motorcraft 80W-90 gear oil (front M210) — 1 qt | Ford / Amazon | ~$16 |
| Differential cover gasket or RTV (if pulling cover) | Permatex Ultra Black | ~$8 |
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.