Report on GM Syncromesh Trans. Fluid

[srgunz]

Well after hearing good things about GM #12345349 syncro fluid I finally went to the local GM dealer and bought two quarts of the stuff. It is specially formulated for all yellow metal syncros. I drained the recommended gear lube that I had in the gearbox this weekend. I have a 78 5speed installed in my 1972. I can report now that I have very happy syncros. I have maybe 80 miles on the new fluid.

The shifting just kept getting better and better the more I drove it. I would absolutly say this is the best this gearbox has ever worked. You can toss it in any gear anytime. It makes no complaints. Also no more notchy shifting feel.

The bottle says it is a partial synthetic. It is definately thinner fluid however the gearbox seems quieter.

Next up is to get that gear lube out of my LSD and put in a synthetic type.

[BillZ260]

Cool, thanks for the input. Let us know if it keeps you happy!

[Cruez]

I put it in my '91 SHO a couple of years ago and it definately seemed to shift smoother.. synchros seemed to work better also.. not the cheapest stuff in the world but it seemed to work.

 

One thing to watch out though... check to make sure that it is not leaking.. my old fluid didn't leak but the synchromesh seemed to have disappeared slowly... must have been while it was hot because there was never any in the driveway.

[240hoke]

Ive been running straight syncromesh for a couple years, its great stuff!

[olie05]

Originally Posted By Mitch

A while back we had a discussion about the "miracle" gear oil' date=' GM Synchromesh. It's been a miracle cure for bad synchros in our old Toyota trannies. It seems that it's even better then Redline MT-90 in this regard.

 

So, GM needed a new oil a while back. They put out the spec (9985648) and a bunch of companies responded with a product that was what GM needed. One of them OEM's it as GM Synchromesh (GMS) in one-liter bottles as part number 12345349.

 

How many of you knew that Pennzoil makes a comparable product? (Thanks Craig) As a matter of fact, it says on the back of the Pennzoil Synchromesh bottle, ".... specifically formulated for synchromesh transmissions used by General Motors requiring General Motors Part No. 12345349 (Specification No. 9985648) Synchromesh Transmission Fluid.... In other words, pretty much the same as GMS.

 

You can buy Pennzoil Synchromesh at some (or maybe most) Autozone stores. Unfortunately, most of the employees have no clue that they have it on the shelf. It sells for $4.99/quart, making it quite a bit cheaper than GMS, despite the fact that you get a little less per bottle in the Pennzoil quart.

 

I bought five bottles a few days ago (more than one car at home). I drained out the MT-90 in my '91 N/A tranny and replaced it with Pennzoil Synchromesh. (My third gear synchro has often been grinding when shifting ever since the autocross a few months ago.) One drive down the road and it was obvious that the problem is pretty much gone. And as most of you that are already familiar with GMS know, "healing" the synchros doesn't typically happen instantly, it takes some driving to clean up the issues.

 

OK, now for the "bad news". From the spec sheet (http://www.pzlqs.com/Tech/Pdsheet/D...romeshFluid.PDF) on Pennzoil Synchromesh, "Pennzoil Synchromesh Fluid is NOT intended for all GLS Applications....).

 

So, what's this mean? According to the Pennzoil tech rep namded Randy that I talked to at the other end of their 800 number, a transmission with "yellow metal" synchronizers should have gear oil with a certain sulfur content. GMS and PS do not have sulfur content, as they are full-synthetics.

 

According to Randy, "the absence of sulfur in the gear oil will lead to long-term corrosion of the yellow metal parts of the synchros, pretty much destroying the tranny." And pretty much undermining the whole reason for putting synchromesh in there in the first place, to fix synchro issues. Note that Redline MT-90 is a GL-4 rated gear oil, hence according to Randy, it has some sulfur content.

 

More bad news. Let's say that you just bought a nice new '93+ MR2 LSD tranny and you want to preserve it's life, so you fill it full of GMS or Pennzoil Synchromesh. BAD IDEA, since it's operational life will be shortened.

 

So, some GOOD NEWS. If you have an S-54 tranny ('91-'92 N/A) that was designed to run Dexron II ATF fluid, it will be very happy with GMS or PS.

 

If you have a '91-'92 Turbo, I'm not really sure what the correct oil to use would be, but I suspect that it's better to use synchromesh than to grind up your synchros.

 

 

Possible solutions:

 

1. Put MT-90 in there instead. (Or maybe Royal Purple, or Amsoil, or whatever else has at least a GL-4 or GL-5 rating.)

 

2. Blend it with MT-90, perhaps 2:1 (one part MT-90).

 

 

There's more to be learned on this topic, and I welcome corrections and clarifications from all of you.[/quote']

 

also...

 

Originally Posted By Mitch

The issue is with LONG TERM life. Trannies aren't going to crap out this week because of the wrong oil. I don't have a compatibility answer yet on MK1 trannies' date=' but they may be just fine anyway. It's not like they will be damaged by Synchromesh, they just won't necessarily have the protection that is needed for longevity.

 

 

EDIT (New info).

 

After more conversation with Randy:

 

Trannies built with "yellow metal" synchronizers (copper and/or bronze) need a GL-4 or GL-5 rated gear oil. These oils use sulfur as a pressure and wear additive. Synchromesh is NOT GL-4 or GL-5. (It's actually "GLS".) Also note: GL-5 provides twice the pressure protection of GL-4. Synchromesh WILL NOT provide the pressure and wear protection to the synchronizers in the long term.

 

 

Ok, here's how it goes with compatibility:

 

The MK1 SC tranny calls for GL5, the non-SC tranny calls for GL-4.

 

The '91 N/A tranny was designed for ATF, so it likely is fine with Synchromesh. Later on, Toyota wised up and specified GL-4 (I think).

 

The '91 and '92 Turbo tranny calls for a GL-5. Synchromesh doesn't have the sulfur, so it's a bad idea for the long run, despite the intial fixes it appears to do. It probably works better because it is simply more slippery (than even MT-90) and provides excellent crush protection.

 

The '93+ Turbo tranny calls for GL-5. DON'T use it in your nice new tranny!

 

Blending is not "officially" recommended by Randy, but who knows? (You would actually be reducing the potential protection.)

 

So, what are we to do? Pennzoil makes a full synthetic GL-5 rated gear oil that should, in theory, do the same tricks that Synchromesh does, AND it provides the correct protection. Supposedly, about $5 or $6 a quart at NAPA (at least; other vendors, we don't know yet). Here's the stuff:

 

http://www.pzlqs.com/Tech/Pdsheet/D...AE75W90GL-5.PDF

 

So, somebody put this in your ailing tranny and LET US KNOW!

 

 

 

Synchromesh has proven many times to be superior to MT-90.

 

I damaged my third gear synchro autocrossing this fall and it hadn't been reliable since. I drained out the MT-90 and replaced it with Pennzoil Synchromesh about two weeks ago. The first drive down the road proved that it was a "better" product.

 

But remember, I have an N/A tranny, it's OK to use it in mine.[/quote']

[srgunz]

I too think that a long term test is needed on the Datsun gearbox. I hope to do that. However, I can only drive my Z 6months out of the year where I live.

My one concern is if the Datsun seals are compatible with this fluid.

[240hoke]

All my transmissions have been from junkyards and have had questionable backgrounds. Ive been using GM syncromesh in my Datsun trannies. I used it in my 260z with a 280z 5spd for over a year after switching from redline. It did well and helped to smooth it out. My current transmission is a junkyard 280zx tranny, Ive run syncromesh in it since Ive had it on the road, shifts great and is holding some good power . I think my motor will kill it before any gear lube can.

[JMortensen]

I wonder if that's the same stuff as BG's synchromesh. Heard goods stuff about the BG fluids from DSM guys, and I've used some of their other additives myself with good results.

 

EDIT-Checked a bit online and it looks like Pennzoil, Quaker State, BG, and GM Synchromesh are at least similar if not the same thing.