Thanks, Chickenman

Interesting. When I bought my first Z in 1973 (a used 71), it came with a crunchy trans. No bearing noise, just crunchy shifts. Drove it about a year before tearing it down to replace the synchros and bearings. When I got it apart everything was pristine.??? Synchros looked new. Having new parts on hand (OEM) I reassembled the trans. using the new parts. Problem solved. I have never known what was wrong with the original synchros. Now I suspect it could have been a change of gear oil (different brand?) that cured the crunchy shifts. I drove the car another 20 years and never had synchro problems again. I did refresh the trans, years later for bearings. I'm curious, was there a great difference in gear oil back then? I likely used Kendall gear oil as the replacement.

I don't recall there being the selection of manual Transmission oils back then, compared to what we have now. Most of the time  75w-90 was a pretty standard spec. That is molasses compared to what is run these days in some Transmissions. As fuel efficiency became more and more important, friction reduction by using thinner and thinner oils in both the engine and drive-train became more important.

 

The development of Synthetics, Synthetic Blends and special additive packages revolutionized both Engine and Drive-train lubrication. You could run much thinner oils ( which helps Synchro engagement ) without sacrificing gear protection with the newer Technology oils

 

Fresh brand new Synchros ( Brass ) do make a difference though. You can slice your finger open on the grooves they are so sharp. Old synchros, while they looked fine to the naked eye, often had dull edges. That adversely affects shift quality.

 

I suspect some transmission fluids were better than others in the 70's and early 80's, but the Oil Technology wasn't nearly as good as it now is. Probably more " Hit and Miss " back in those days. Just my .02c

Edited June 25, 20159 yr by Chickenman

I found the AC Delco product on eBay for $13.43 each, free shipping.  Need 1.5 liter for the tranny.

 

This is a great thread, thanks for all the details on the friction modified fluids.

What Jim said ^^^^

Thanks

So whats been said in this thread conflicts a bit with what I have read elsewhere.  Specifically about the GLS synchromeshs and their sulfur content (or lack of) It appears a lot of the statements may refer to information from a Penzoil tech about their equivalent product, but there is also mention of GM's version as well.

 

http://forums.hybridz.org/topic/46974-report-on-gm-syncromesh-trans-fluid/

 

Clearly people will have differing beliefs about something like this, and Chickenman certainly seems like an informed party on this topic. Just wondering if there is any more clarity I could get before I pull the trigger on the Friction modified stuff vs just Redline MT90.

 

Thanks all

Are you worried about synchro durability or shifting performance?

 

You can't use the GL-5 spec. to determine if the sychros will fail.  There may be a correlation of failure to a few GL-5 fluids, but there is no proven cause-effect relationship.  Many GL-5 fluids cause no problems at all with "yellow metal" synchros.  The GL specs are about performance, not chemistry.  The focus on GL-5 is an attempt to find a "code" that will predict behavior but it doesn't work.

 

So that leaves actual experiences as one rational reason to choose a certain fluid.  It looks like a few people here have tried a few different fluids.  Many people have used Redline's MT-90.  I've seen other posts about the GM/Pennzoil formula Synchromesh.  Swepco 201 looks good, besides one report of synchro failure in a race car, during a race.

 

If I were you I'd start a new thread asking what people are using and how it's working.  Take a poll.  Forget about GL-4 or GL-5 or GLS.

 

Don't forget to read the label or any factory literature about fluids also.  If the manufacturer is reputable and says that their fluid will work with "yellow metal" synchros, odds are good that it will.  Otherwise they wouldn't be reputable.

It began with simply the question about which tranny fluid would best suit my 71 4speed, since I am refreshing it after sitting in storage for 30 years prior to my ownership. I don't have any seat time with the transmission yet as there are other systems needing work before I can drive it, but if there was a defacto fluid I would replace it during my process and hopefully minimize the issues I might see in the future.

 

My personal preference would be towards shifting performance, which is why I was leaning towards the Synchromesh. There are only 29,000 miles on the car, so its very young. And someday I would probably swap in a 5 speed, so even if there was a slight degrade over time it probably would never come onto my radar at least with the 4spd.

 

That said, I plan to own the car for a long time to come, so I don't want to regret a hasty/under informed decision I make now.

Edited December 3, 20159 yr by DaveR