Tried searching, did not find what I was looking for. I am doing a full fluid change on the beast. FSM says 75w-90 for my manual transmission. Having a hard time finding it. Lots of 80 and 85w-90 and 85w-140 out there, just cant seem to find the 75w-90. Would there be a significant impact on my AX-15 if I were to use a different viscosity range than what the FSM says?
In case I have not updated my signature in a while, 1994 XJ, Manual tranny, 231 t-case, dana30/chrys8.25, Rusty's 3" with 31's.

 

I don't see too many of these threads on the XJ boards, but I also have a 95 YJ and these threads are all over the place. I have used this: in my YJ for the last 75k miles (it has 150k on it now and I'm owned it since new). It is GL-4 and available at NAPA. The YJ owners manual is incorrect and specifies GL-5, I don't know about the XJ manual. You do not want GL-5 unless it specifically says yellow metal safe. Redline MTL-90 is another popular option. I have seen many threads that talk about the official recommendation to use 10w-30 motor oil. All I can offer is, the Stalube has worked great for me, it's readily available and is GL-4 so yellow metal safe for the synchros.

 

The synthetic Valvoline gear oil I use in my diffs is 75W-90. VV975
Was curious if they make one that is not. Pretty sure VV820 is but also says for Limited Slip diffs like the VV975 says.
Not sure how any of this effects what you guys are trying to do though.
My '00 is an auto. LOL.

 

You shouldn't use regular gear oil in the ax15 because it has additives in it that aren't good for the syncros. You have to use a special gear oil for manual transmissions or many people use motor oil. I am running 15w40 Diesel oil in mine and it shifts great.

 

I tried the Napa house brand GL-4 after I rebuilt my AX15, intending to change it in a week to Redline. It shifted so bad I though I screwed up the rebuild. I put a 50/50 mix of Redline MT and MT90 in, and it shifted beautifully after that. Ditto on all the other posts that you want something meant for manual transmissions that is safe for the brass synchros (ie doesn't use sulfur based EP additives). Never run motor oil myself, but many swear by 10W-30 synthetic motor oil (roughly the same viscosity as the prescribed gear oil.

 

The FSM does say 75w-90 GL-5. I did go over and look at the Wranglers forums and many are using motor oil as well as the Redline product.

 

I actually have the original owners manual that came with my XJ 75 90 gl5. So I'll just do it everybody says just make sure read the label make sure it's good for soft metals

 

https://www.naxja.org/forum/archive/...t-1048661.html

 

I know you probably don't care, but I used Dex/Merc and 1/2 can of STP in my Muncie 4-speed when I raced my Camaro. Shifted slick and never had problems with the syncros.

 

I believe Chrysler now recommends using 10W-30 motor oil.

 

The FSM for my 2000 XJ says to use 75W-90 GL-3 gear oil in the AX15.

 

That said I recently drained the AX-15 on a '91 XJ and refilled with Mobil 1 10W-30 and it is hard shifting into first and second when hot. I am not sure what fluid was in there when I got it or if I have other issues. The proper manual transmission fluid seems to be one of the more confusing issues out there. To make it worse it seems that Chrysler made an error in some manuals by identifying a GL-5 rated fluid. It seems that many GL=5 fluids are not recommended for use with synchronizers as additives can degrade brass and related “yellow metals”. The original GL-3 or GL-4 gear lubes have largely been discontinued and replaced by commonly available GL-5 lube that often indicate downward compatibility for GL-3 and GL-4 but is not safe for yellow metals. OK for differentials but not for most manual transmissions. The safest approach seems to be to use a fluid known to be safe for yellow metals. Engine oils should fit that but viscosity may not be ideal over a full temeprature range.

 

If you want a dinosaur's opinion, straight 90W GL-5 has been used in manual transmissions with brass synchros since there WERE brass synchros, like in the 40's! I don't understand how it's "incompatible with yellow metals."

 

I doubt GL-5 has been used in transmissions since the 40's. I believe it is a relatively modern spec. API GL-5 is based on ASTM D-7450, a modern spec. Each GL version of GL-1 to GL-5 was released to meet increased Extreme Pressure (EP) and temperature demands of differential gears and similar, not transmissions with synchros. To meet EP gear requirements higher levels of sulfur and phosphorous based compounds were used. That is fine for steel differential gears but brass components do not like the higher levels of EP additives such as in GL-5. Apparently some GL-5 additatives can result in increased brass synchro wear in transmissions. I have done a fair amount of research and have some additional I want to follow up on. I will post results of my findings along with references later.

 

Get some Pennzoil syncromesh and call it a day. I ran that stuff in 3 different AX15s and never had any issues.