So I couldn’t seem to find any Pennzoil synchromesh or redline at my nearest auto stores that were

75w-90 so I picked up this valvoline 75w-90 GL-5 like the owners manual recommends but I don’t know if this is a good choice or if it’s safe for yellow metals has anyone used this stuff?

 

Yellow matals and the phosphorus in GL-5 don't play well together.

The original spec was GL-4.
There's a thread here from ages ago about the typos in the owners manuals VS the factory service manuals.

The parts store orders in synthetic GL-4 for me when I ask them to.

 

That stuff has limited-clip additive, whcih you don't want. GL-4 isn't much better for the EP additives. Syncrhomesh, Redline, or synthetic 10W-30 motor oil would be a better choices.

 

Okay I think I’ll go out and get some synchromesh just out of curiosity does it need to say 75w-90 on the bottle and also why does the manual recommend gl-5 if it’s not good for it?

 

Do a site search. There's a thread from years back where the found and discussed the typos and copy paste errors in the service and owners manuals.

 

GL-5 has, among it's ingredients, phosphorus as an anti wear additive. It's king for a lot of applications.

In rear diffs heat AND pressure make the phosphorus form a slippery glaze on the steel gear teeth faces where the steel ring and steel pinion touch each other. The glaze is only as thick as an Angels breath but it's enough.
If it gets worn away on the next redline launch, no sweat because it builds up again in normal running.

Ok now, Phosphorus and the yellow metals in transmissions is a little different story. When under pressure phosphorus doesn't just build up, it moleculerly bonds to yellow metals.
When that phosphorus candy coating breaks away it takes some of the yellow metal with it.

Gear & shaft bushings and synchro blocking rings are where brass and bronze are used in transmissions.

Something to bear in mind if you're keeping your Jeep for a while.

 

Red Line MT-90 is like $24/qt at O'Reillys and they seem to get it the fasted out of any of the other big-box stores. Otherwise if you have patience, it is like $17/qt via Amazon.com. Pennzoil synchromesh will work but from my past history of owning sticks, the MT-90 is the gold-standard. Certainly made my VW's shifting buttery soft lol.

 

The EP additives also reduce the friction between the synchros and the gears, which reduces their ability to synchronize the gears when you're trying to shift. Manual transmission specific oils have a much better friction profile.

 

Don't put that in there. I used 15w40 diesel motor oil in mine and it works great.

 

You're the first I've heard of using 15W-40. Rumor has it the Jeep dealers are filling with 10W-30 now.

 

Shifts smooth as butter with it, no problems in 5 years.