AX-15 holds 3.3 qt of synchro-safe 75W-90 GL-4 or ATF per the original FSM. Avoid GL-5 hypoid oils which attack the brass synchros.
The Aisin AX-15 five-speed was offered behind the 4.0L from 1989-1999 (Renix used the BA10/5 'Peugeot' instead, which has its own fluid quirks). Capacity is 3.3 quarts (3.1 L). The original Jeep TSB called for 10W-30 motor oil, later updated to ATF and ultimately to a 75W-90 GL-4 (or GL-4/GL-5 dual-rated such as Redline MT-90 or Pennzoil Synchromesh). The critical point is that straight GL-5 gear oil contains active sulfur that pits brass synchros and produces notchy 2-3 shifts when cold. Fill until fluid trickles from the side fill plug on level ground. Drain plug is on the bottom of the case, fill plug on the driver side; both are 17mm hex (Allen). The BA10/5 in Renix XJs uses 80W or 75W-90 GL-5 per its French spec and is far less tolerant - confirm transmission before ordering fluid.
| Part | Vendor | Est. price |
|---|---|---|
| Redline MT-90 75W-90 GL-4 (1 qt x4) | Summit | ~$60 |
| Pennzoil Synchromesh (alt) | auto parts | ~$40 |
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.