Why use ATF in a manual transmision?

[Bonnewagon]

I have always used 90 weight gear oil in my old school manual transmissions with no problems. I put some in my '99 Sunfire Getrag 5 speed and I got gear clashing. Too thick. Switch to Synchromesh and all was well. I see some manual transmissions even use ATF. Why? I ask because I will be putting my A833/NP440/MY6 4 speed w/OD transmission into service eventually and I am seeing all sorts of reccomendations as to which lube works best. Even the Mopar factory manuals disagree. The most popular opinion is regular 90 weight or so but yet some swear by ATF and other blends of lubes. I would expect such thin oil to cause wear issues as well as sychronizer malfunction. Can anyone explain this?

 

[1evilregal]

Don’t know, I always thought that it was due to tighter tolerances and flow ability...

My old 87 dodge truck called for 10w30 in it’s manual trans....

 

[joesregalproject]

The 5 speed in my '95 F150 calls for ATF. Other people have used gear oil, various 30 weight oils, And Pennzoil Synchromesh with varying results. I just stick with the recommended ATF.

 

[DRIVEN]

Maybe over the course of several hundred thousand miles wear may be an issue. I said in your other thread that Synchromesh was what I found worked best in my A833s. My working theory is that those larger (heavier) gears need a less "slippery" oil to help slow them down when the synchro engages the cone. That's why some manual transmissions are more likely to grind when cold but shift smoother once the oil gets warmer and thinner.

I realize there is conflicting specs on the A833/MY6 because I ran into the same thing years ago. When I used 90w I had to shift slow or it would grind. With ATF it shifted like butter but was noticeably noisier even inside the car. I ran across a couple forum posts that mentioned Synchromesh and found it to be the happy medium. Figured it probably had a better additive package than regular 90w or even something like a 30w.

My advice is to experiment. They're pretty forgiving transmissions so you could probably use just about anything for a short while without doing any damage.

 

[Bonnewagon]

Driven I remember your advice. That is one of the reasons I asked again. So many opinions out there. The Synchromesh worked wonders in my Sunfire but that is what was spec'd. The manual even explained that there is a gear "spin-down time" involved and the thick 90 weight extended that to where shifting suffered. My fear is wear, as some have reported, with the ATF. I even found a report that gear clashing with the ATF could be cured with heavier lube. I bought this from Spidereyes455 and he used regular gear oil but he took the trans out due to a leak so I don't know if he had long term issues. . You are the only one I know that has a lot of experience with these transmissions so I value your opinion over the rest.

 

[olds307 and 403]

As said, ATF, 5/10W30 and 80w or 90w along with synchromesh recommended for different manual transmissions. What weight is Synchromesh? I know our company makes a 50W synthetic gear oil for transmissions and differentials.

 

[Bonnewagon]

I found this on the interweb:

I suspect that most older manual transmission were designed to work well with conventional gear lube and some may have even called for 30w motor oil. But I've seen Chrysler 4 speeds run with A/T fluid with no ill effects. I believe Chrysler even recommended ATF for better cold weather driveability. But these trans used conventional brass blocker rings in the synchronizer assemblies.

But at least some more current model trans have blocker rings that look like metal stampings with a wear surface that looks like the organic clutch material used on the bands and clutch discs of an automatic transmission. I think this is where you see the recommendations for thinner synthetic lubes. The organic faced blocker rings are better able to cope with the increased lubrication properties of the synthetic lubes.

But I'd be cautious of using full synthetic lube in an older trans with brass blocker rings. The extra slippery lube can make it difficult for the brass rings to get the needed "bite" on the synchronizer hubs. You might end up with some gear clash even on a new or freahly rebuilt transmission. The increased slipperiness of the synthetic fluids, while beneficial for most of the transmission, actually over-lubricates the older style synchronizers.​

 

[olds307 and 403]

Similar to clutch style posi diffs, synthetic isn't for everything. I wonder if something like a 5W50 Motorcycle oil without friction modifiers would work?

 

[Bonnewagon]

This is good too but it won't paste. http://stealth316.com/misc/pennzoil_gearoilsummary.pdf I just discovered that I should NOT have used GL-5 in my Saginaw. GL-4 is correct and the GL-5 has sulpher that will attack the brass synchros. It is for rear end gears. Also a good read: https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1231182

 

[DRIVEN]

Yes, GL4 for brass synchros.