Im fairly sure the Chrysler TSB on fluid for the AX-15 says 10w40 bynthetic, but of course at manual trans temps there's not all that much difference between most 10w30s and 10w40s. I used RP 10w40 for about a year, and it worked well most of the time. On the hottest of summer days, my AX-15 tends to "block" first for a long time while the input shaft spins down (and its not a dragging clutch, it passes the test for that), so at my next drain/refill, I decided to drop in M1 15w50 synthetic. That works the best yet, better even than Redline MT-90. I guess it could possibly make shifting a little sluggish in winter if it ever got colder than the mid 20s here... but it only gets THAT cold once or twice a year.

FWIW, the 10w40 came out looking brand new. I had used Royal Purple 10w40 synthetic, and it came out of the gearbox still purple after about 15,000 miles and over a year, no gritties or shinies in the fluid and a minimum of beard on the magnetic drain plug.

I used Mobil 1 synthetic 10/30 and it shifts perfect. Dump that fluid before you do any damage to the synchro's or something.

All of this is good information and Amsoil is a good product if you can get it easily. I figured it'd be time for an update on how the Royal Purple 10W-40 is doing in my transmission. As I said a few months ago my shift's are smoother and Reverse ingages just as easy as It did when I put it into gear. I also had an odd squeek noise (most likely from my slave cylinder) when I pushed the clutch pedal in. It still has not returned and my shifts are still smooth and the rough 1-2 shift seems to be alittle more cushoned now I'm running the RP. Just thought I'd post an update for anyone who is interested.

Hi Joe,

I love 5-speeds, but frankly the one used in pre-2000 Cherokees isn't all that great (Aisin AX-15). It is better than the AX-5 and a Pugeot that Jeep used in the 80s, though. My main gripe with it is that they changed the synchros in about 1996 so that it REQUIRES GL-3 oil, just as GL-3 oil was becoming unobtainium. It also needs a 90-wt oil, so you can't use ATF. I tried Redline MT-90 (GL-4, about the safest gear oil I could find) and within a few months started getting blocked shifts to first gear and other synchro quirks. I went to a Chrysler dealer service department and basically said, "Ok, smart guys, what do YOU use in one of these when it comes in for service?" They pulled their supercede list and TSBs and showed me a paper trail where Chrysler engineers recommend 10w40 synthetic engine oil for the AX-15. So I tried that, and darned if it isn't working well again.

That said, its actually a pretty nice-shifting unit, and seems to go a long time without trouble for the average driver. Just not super beefy and I gather it sometimes fails in harder offroading. It has your typical Japanese truck transmission super-noisy reverse gear, also. In 2000, Chrysler switched the Cherokees to the New Venture Gear NV-3550 for the last 2 years of production (and its also used in Wranglers IIRC). MUCH tougher transmission, uses ATF IIRC.

Mine had a clutch/throwout bearing replacement at ~60,000 miles (previous owner) because the rear frost plug was leaking coolant and the transmission had to come out anyway. So as of now my clutch "only" has about 75k miles on it. So far so good. The previous owner had a 93 Cherokee with an AX-15 prior to this one, and its clutch was still going fine when he sold it with around 130k miles.