Drop the 13mm drain plug, spin off the filter on the passenger side, refill with 6 quarts of 10W-30 (4.0L) or 5W-30 (4.0L in cold climates), and tighten everything to spec — first oil change on a YJ takes about 30 minutes once you've done it.
The AMC inline-six in the YJ runs forever on clean oil and falls apart on neglected oil. Both engines — the 4.2L 258 (1987–1990) and the 4.0L HO (1991–1995) — share an oil-pan layout, a side-mounted spin-on filter, and a forgiveness for cheap conventional oil if you change it often. The 4.0L holds 6 quarts with filter; the 4.2L 258 holds 5 quarts with filter. Use 10W-30 for desert and summer, 5W-30 if you see freezing temps. Full synthetic is a real upgrade after 100k miles when the rings start letting a little blow-by through.
Mopar's filter for the 1991–1995 4.0L is **5281090AB** (M0-090 on some packaging). The 4.2L 258 takes the older **33004195**. Wix, Bosch, and Mobil 1 all make a fitting alternative — anything with the right thread and gasket size will work, but the Mopar filter has the strongest anti-drainback valve, which matters on a side-mounted filter where oil can siphon back into the pan on shutdown.
Warm the engine for five minutes before draining — warm oil carries more suspended particles out with it. The drain plug on the pan is a 13mm hex head; the torque spec is **22 ft-lbs**. The filter sits horizontal on the passenger side of the block above the starter — a swivel-handle filter wrench or the cap-style wrench made for this filter size makes it manageable from the side. Spin the new filter on by hand until the gasket touches the block, then another three-quarters turn. Don't gorilla it — the gasket does the sealing, not the torque.
After refill, run the engine for 30 seconds, shut it down, wait two minutes, and recheck the dipstick. Top up to the upper mark. Drive it around the block and check for drips at the filter and drain plug. Log the mileage. The AMC six wants fresh oil every 3,000 miles on conventional or 5,000 miles on synthetic — the engine will thank you with another 100,000 miles of service.
| Part | Vendor | Est. price |
|---|---|---|
| Mopar oil filter (4.0L, 1991-1995) | Mopar | ~$9 |
| Mopar oil filter (4.2L 258, 1987-1990) | Mopar | ~$9 |
| Mobil 1 5W-30 full synthetic, 5 qt | Mobil | ~$32 |
| Valvoline conventional 10W-30, 5 qt | Valvoline | ~$22 |
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.