I have a T5 in my 85. I changed mine back in the Winter, to Syncromesh. It made a HUGE difference- shifts much smoother, especially in the cold. Shifts were notchy before, and the cold affected downshifts quite a bit until the transmission warmed up. It had gear oil previously, although I don't know what weight. Regardless, I won't run anything other than Syncromesh, now.

 

My neighbor has a T4 in his CJ. He claims very good performance from synthetic ATF from Red Line Oil.
https://www.redlineoil.com/d6-atf

UTN

 

hello

i tried syncromesh but it did not shift well especially down shifting. im in 80F+ weather most of the year and only feels like a month of cool weather. i am running a 90w GL4 oil that i got from a truck shop that is used in tractor trailer automated manual transmission. a lot of research that the jeep T4 and T5 are a NON World version. mostly that means there is no fine roller bearings between the gears and the main shaft. Tremac who now makes the T5 transmission still recommends a gl4 oil for the non world version.

i would try different ones and see which works best for you.

oldschool

 

I use Pennzoil Syncromesh as well in my T5 and had it in my previous T4 also. Good stuff and shifts are as smooth as butter at any temperature.

 

You will find that this very question is a touchy topic. There are many opinions, conflicting manufacturer recommendations and sooner or later, someone will tell you "Use XX or it will ruin your transmission."

I can only speak from my personal experience with my CJ and my S-10 Blazer. When I bought my CJ (85 CJ-7 with the 151 4 cyl) it had 80W-90 in the T4. Most information seems to indicate that the Jeep CJs with the T4/T5 came from the factory with 80W-90 gear oil in the transmission. My 1985 S-10 Blazer had the exact same transmission as the CJ, but the factory installed Dexron II ATF (probably to get better fuel economy numbers). The Blazer transmission was a bit louder than the CJ, but I choked it up to the CJ having plenty of other noises to cover a little transmission noise. I tried Dexron III in both transmissions (mainly because Dexron II is no longer available) and both transmissions performed equally, though after changing to ATF in the CJ, the transmission noise became more apparent.

By this time I was working at an auto parts store and received a training on lubricants from a Pennzoil factory rep. The instructor talked about Synchromesh manual transmission fluid and how it had improved lubrication properties over ATF, was compatible with softer metals used as synchronizers in manual transmissions, (some gear oils are not) and was less viscous than gear oil, so it offered the potential for improved cold weather performance and fuel economy.

I decided to give it a shot in both the CJ and the Blazer. In both cases the transmissions were noticeably quieter than with ATF, and shifting was quicker/lighter than with gear oil in the CJ. I did not see a significant increase in fuel economy, but neither the CJ or the Blazer were really built for that. Cold weather shifting was significantly improved over gear oil and no worse than with ATF. I have continued to use the Syncromesh fluid in T4 transmissions and in the T5 when I "upgraded" the CJ.

It is a moot point now as I swapped an SM420 into the CJ and sold the Blazer, but my recommendation is the Synchromesh manual transmission fluid, YMMV.