what to use to top off Manual Trans, fluid unknown

[bowens]

Hi, I've got a 1991 Geo Metro 5sp, and I need to top off the trans fluid for a couple hundred miles before I change the fluid completely. I don't know what's in there, and I'm wondering what I should use for the top off.

Been doing a lot of research on what should go in there, and have concluded a straight GL-4 synthetic will be best, like Amsoil MTG; in the meantime, can I go with 90w gear oil, or maybe 10w30 motor oil (synthetic or regular)? I've got both on hand. We're talking about probably 1/2 quart of the 2.5 capacity; 75W recommended in the factory manual.

 

[TallPaul]

Used to be able to run motor oil in a transmission but now with the zddp so low, I would not trust it.

Why not just change it out now?

I run Redline MTL in my Ranger pickup.

 

[bowens]

what's zddp? I need to test some things out before I commit to the $50 worth of fluid...
Originally Posted By: TallPaul
Used to be able to run motor oil in a transmission but now with the zddp so low, I would not trust it.

Why not just change it out now?

I run Redline MTL in my Ranger pickup.

 

[bepperb]

50 dollars worth of fluid?

I'd go down to a Honda dealership and get two quarts of MTF for six bucks each, drain and fill and the car will be in the boneyard before you ever need to think about this again.

 

[bowens]

this is a restore job, so the boneyard is the opposite direction. Also, from what I can gather, the Honda product has viscosity like motor oil, so maybe it's ok for the top off, but my starting point is 75W for the complete change.
Originally Posted By: bepperb
50 dollars worth of fluid?

I'd go down to a Honda dealership and get two quarts of MTF for six bucks each, drain and fill and the car will be in the boneyard before you ever need to think about this again.

 

[salv]

nissan manual transmission fluid is 75w85, hyundai manual transmission fluid is also 75w85.

top off with cheapo 80w90, coastal, supertech, etc. if you aren't going to change it.

I have succesfully used synchromesh in even 80w90 applications.

 

[sciphi]

Gear oil and motor oil are two totally different viscosity scales.

A 75w-90 gear oil is very roughly equivalent to a 10w30 motor oil in terms of hot and cold viscosities.

Think it through. That's why they recommend either 75w-90 gear oil or a 10w30 motor oil. Similar enough viscosity to where the transmission won't tell the difference viscosity-wise.

I'd top off with the 10w30 dino oil, then put in a synthetic 75w-90 like Amsoil or the like.

 

[92saturnsl2]

That old girl would have specified 80w-90 GL-4 gear oil or equivalent.

For top-off, I imagine just about anything would suffice, but I'm always leery about how nice different additives play together. 30-40 weight motor oil falls around the same viscosity as the 75w-90 gear oils, so there should be no issues should you decide to add motor oil for a short time.

Synchromesh is more toward ATF in terms of viscosity, around a thick 20 or thin 30 weight. I'd only recommend Synchromesh for applications that recommend ATF.

There's a lot of nice OEM manual transmission fluids out there that would work great. Stay away from GL-5 gear oils as they will wreak havoc on yellow metals, specifically the synchros in the transmission. Ford has a new synthetic MT fluid, very similar to a GL-4 rated 75w-90, but it's about $18 a quart.

 

[bowens]

thanks, everyone

 

[bepperb]

Originally Posted By: bowens
this is a restore job, so the boneyard is the opposite direction. Also, from what I can gather, the Honda product has viscosity like motor oil, so maybe it's ok for the top off, but my starting point is 75W for the complete change.


I hope it goes well. My point wasn't that this is headed to the junkyard, but that if you change the MTF now you can reasonably expect 100,000 miles out of it and really not consider it a maintenance item going forward. If it were mine, and I lived in your milder climate, I'd just get the Honda MTF or some Pennzoil Syncromesh and call it a day. Either are a better product than what was original on that car.

 

[mechtech2]

Top it off for a few hundred miles?
Use anything. ATF, motor oil, or any trans fluid.
Really.
Now when you actually refill it for future service, that is another story...

 

[MolaKule]

Quote:
Synchromesh is more toward ATF in terms of viscosity, around a thick 20 or thin 30 weight. I'd only recommend Synchromesh for applications that recommend ATF.



This is a common myth.

Synchromesh manual transmission fluids may come in any viscosity from 7.5 St, an ATF viscosity, to 14.5 cSt, a 75W90 viscosity.

https://bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1231182#Post1231182

For a used Metro, I would start out with something like a Amsoil MTG or Redline MT-90.

 

[92saturnsl2]

Originally Posted By: MolaKule
This is a common myth.

Synchromesh manual transmission fluids may come in any viscosity from 7.5 St, an ATF viscosity, to 14.5 cSt, a 75W90 viscosity.


Excuse me, who says?

Synchromesh is a special fluid required only by a handful of transmissions from GM and Chrysler. I believe it was a GM transmission that introduced the requirement when some bearings and internal parts failed to receive adequate lubrication while using the standard gear oil found in nearly all manual transmissions at that time.

You seem to be grouping all other manual transmission fluids into that category, and that's not correct. Just because an oil company has added the word "synchro" to the name of their MTL, doesn't mean it's a Synchromesh fluid. Synchromesh does not mean any MTL that's safe for synchros, it's the special fluid required by those few select GM and Chrysler transmissions.

Usually the word "synchro" is in product names and literature only to imply that the product is safe for synchros, as some GL-5 and other gear lubes are corrosive to the soft metals in the synchros.

AFAIK, there are only a handful of true Synchromesh fluids: Pennzoil Synchromesh, GM Synchromesh, GM Synchromesh w/ Friction modifier, RP Synchromax, maybe one or two others. You'll find that the viscosity is a LOT closer to ATF than a 75w-90 gear oil. While Synchromesh fluids can generally be used in transmissions that specify ATF, the reverse is not recommended because synchromesh fluids lack the EP additives and viscosity found in GL-4 type MTLs and gear lubricants.

All those examples with the higher viscosities you gave in the link are manual transmission fluids that are safe for synchros and nothing more.

 

[mechtech2]

I'm sure Molakule appreciates your information...

BTW... Do you know what his field is? What his profession is?
And that he also had a company that produced ultra fine trans lubes?

 

[jaj]

Originally Posted By: 92saturnsl2
Originally Posted By: MolaKule
This is a common myth.

Synchromesh manual transmission fluids may come in any viscosity from 7.5 St, an ATF viscosity, to 14.5 cSt, a 75W90 viscosity.


Excuse me, who says?

Synchromesh is a special fluid required only by a handful of transmissions from GM and Chrysler. I believe it was a GM transmission that introduced the requirement when some bearings and internal parts failed to receive adequate lubrication while using the standard gear oil found in nearly all manual transmissions at that time.

You seem to be grouping all other manual transmission fluids into that category, and that's not correct. Just because an oil company has added the word "synchro" to the name of their MTL, doesn't mean it's a Synchromesh fluid. Synchromesh does not mean any MTL that's safe for synchros, it's the special fluid required by those few select GM and Chrysler transmissions.

Usually the word "synchro" is in product names and literature only to imply that the product is safe for synchros, as some GL-5 and other gear lubes are corrosive to the soft metals in the synchros.

AFAIK, there are only a handful of true Synchromesh fluids: Pennzoil Synchromesh, GM Synchromesh, GM Synchromesh w/ Friction modifier, RP Synchromax, maybe one or two others. You'll find that the viscosity is a LOT closer to ATF than a 75w-90 gear oil. While Synchromesh fluids can generally be used in transmissions that specify ATF, the reverse is not recommended because synchromesh fluids lack the EP additives and viscosity found in GL-4 type MTLs and gear lubricants.

All those examples with the higher viscosities you gave in the link are manual transmission fluids that are safe for synchros and nothing more.


Amazing! I couldn't have said it better myself. Except for the bit about what Synchromesh fluids are for. Synchromesh fluids, most of them, are based off of GM's spec for their manual transmission. Since then, though, a bunch of new transmissions and fluids have come to market, all of which bear use that name correctly.

Synchromesh fluids need a combination of the correct viscosity characteristics and friction regimes, otherwise they won't shift properly or work at all the temperatures specified for the transmissions they're intended for.

And, yes, Molakule has forgotten more about gear oils than the rest of us put together could learn if we went to grad school.

 

[92saturnsl2]

Originally Posted By: mechtech2
I'm sure Molakule appreciates your information...

BTW... Do you know what his field is? What his profession is?
And that he also had a company that produced ultra fine trans lubes?

Not sure what the point of this is, does he want a congratulations, a pat on the back?

Do you know what my field is? What my profession is? No, and I don't care to share it because I don't get my jollies from an internet minion parroting everything I say. To each their own.

 

[MolaKule]

Quote:
Synchromesh is a special fluid required only by a handful of transmissions from GM and Chrysler. I believe it was a GM transmission that introduced the requirement when some bearings and internal parts failed to receive adequate lubrication while using the standard gear oil found in nearly all manual transmissions at that time.

You seem to be grouping all other manual transmission fluids into that category, and that's not correct. Just because an oil company has added the word "synchro" to the name of their MTL, doesn't mean it's a Synchromesh fluid. Synchromesh does not mean any MTL that's safe for synchros, it's the special fluid required by those few select GM and Chrysler transmissions.



I disagree my friend and I think this is simply a case of misinformation in which you are somehow mixing historical information with application.

Any manual transmission fluid from 4.0 cSt to 25 cSt that has it's application in manual transmissions with Synchronizer assemblies is a synchromesh fluid.

Using your logic, any manual transmission fluid I use for Honda's, VW's, and Toyota's (which have synchronizer assemblies) cannot be a synchromesh fluid.

A Manual Transmission synchromesh fluid is a fluid that operates in transmissions with a mechanical sychromesh assembly for matching engine rpm to the rear wheels.

The term "Synchromesh" is rightly applied to an Application, not to make of vehicle, historical bloodline, or viscosity. A Synchromesh fluid is rightly applied to ANY manual tranmisson fluid with Synchronizer assemblies.

 

[92saturnsl2]

Originally Posted By: MolaKule
A Synchromesh fluid is rightly applied to ANY manual tranmisson fluid with Synchronizer assemblies.


I stand my ground. Synchromesh fluid is Synchromesh fluid, just the same as Dex/Merc is Dex/Merc and GL-4 MTF is GL-4 MTF.

Ask the guy at your local parts store to get you synchromesh fluid and he'll come back with one or two choices. Pennzoil Synchromesh and Royal Purple Synchromax (for the few that have it).

Synchromesh has become the de facto term for a transmission with synchros, but the fluid is still what it always was.

Manual transmissions have evolved such that nearly all incorporate synchros, but few specify Synchromesh fluid.

 

[SOHCman]

For the sake of argument,

Some manual transmissions call for ATF, I sure wouldn't call an ATF a synchromesh.

Manual Transmission Lubricant may be a broader and more correct term for fluids specifically designed for manual transmissions. "Synchromesh" would fall under that category as a brand name of fluid.

"Kleenex" may be the default name for all tissue paper, but is common terminology really ready to call all MTLs synchromesh?

 

[freshcedar]

Originally Posted By: MolaKule

For a used Metro, I would start out with something like a Amsoil MTG or Redline MT-90.


I originally wanted to put Amsoil's 5w30 Synchromesh Transmission Fluid into a Geo Metro. But instead I'm going to use Amsoil's Manual Transmission & Transaxle Gear Lube 75W-90. This is what sold me:

1.) Well, it is the OEM recommendation. There are arguments that it was a different time back then, or a Synchromesh would be harmless and increase fuel economy and shift quality. And it's tempting, because I have heard of people who have changed and put in new 75W-90 transmission fluid, still had terrible shift quality, but had all of that disappear with some synchromesh fluid. That was the best counterargument that wanted to sway me towards using synchromesh, but, who knows. Well if I did ever experience shifting issues, I would try it out. $30 would be a cheaper alternative than a rebuild maybe. Or that would be stupid, I have no idea.

2.)I heard tell that you are some sort of lube professor? I do not need proof.

3.)Something about 75W-90 transmission fluid being comparable to 5W40/10W40 motor oil, according to Redline? Well that's just swell.

So, I guess I'll be going with 75W-90. Who knew I could change my mind because man I love synchromesh.

EDIT: Man, it's killing me. I think I'm going to go with the Amsoil Synchromesh 5w30. What's the harm? So it's slightly thinner than the 5W-40 Redline says 75W-90 is comparable to. Right? And people swear by the shift quality improvement and increased fuel economy. I mean those OEM recommendations were made a long time ago and a lot's changed. A lot of cars you would never suspect are running 5W-20 now, like Mustangs. Thinner doesn't necessarily mean less protection I read somewhere. I mean it's a three cylinder Geo Metro transaxle. I had a Chevy Sprint TURBO that only called for 5w30 motor oil for the manual transaxle fluid, and it ran like a dream after that, and fuel economy may have improved dramatically. It did, but I'm not sure why exactly. (might have just been driving technique or spark plugs (misfire))

Surely 5w30 Synchromesh will do no harm to a Geo Metro transaxle, right? Or am I wrong?