How do manufacturers choose manual trans oil weight?

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I know this is more of an engineering question that most people won’t know the answer to but as the title suggests how do manufacturers go about selecting the oil weight they recommend for their manual transmissions? You have gear oil as thin as 75w all the way up to 75w90, and some even using ATF.

Why does one manufacturer go with 75w and the other 75w90. What makes 75w90 the fluid of choice in say a Toyota transmission, but not a 75w or a 75w80 fluid? And will using 75w really be that detrimental?

Naturally my intuition told me well of course your compact cars use light weight fluids as they don’t see the stresses of a SUV or truck on a daily basis, or tow for that matter. Where as your “heavy duty” truck transmissions need the juicy thick stuff. From my Isuzu days I remember Isuzu recommending 10w30 motor oil in their troopers that used the AR5, while the same transmission under dodge (AX15) used 75w90.

The Mercedes G56 manual transmission. Ram tells you to use ATF in their Cummins powered heavy towing truck where as Mercedes says to use 75w90.

Point is that there isn’t the rhyme or reason that I was expecting. What are your thoughts? I wanted to get other minds in on the subject and get other perspectives.
 
Shift quality for weak wristed consumers moved many to thin fluids

Cooling... improved fluid movement, fluid control, and component cooling....

Wear rate... the transmission still needs to last a certain amount of time

Usage.... expectations of consumer... ask the statisicians

And, MPG.... its just another fluid we can make thinner to save a few ounces of fuel every year without excessively wearing the transmission out before the warranty period expires

Toyota does use a 75w LV fluid in newer manuals that were designed for the lighter grade.
 
The modern trend seems to be for very thin FE Manual Transmission oils. My Mercedes manual takes what is designated as 75W which is approx. 6 - 7 cSt @100 Deg C. I note 75W doesn't make sense but that's what they label it. At the other end of the car the diff takes 75W85 rather than 75W90 again presumably aimed at reducing losses.

So it seems to me they chose to design a transmission with low losses on economy grounds and designed the gear synchros to match the thin oil. ATF would be of a similar viscosity but with different friction modifiers.
 
The Mercedes G56 manual transmission. Ram tells you to use ATF in their Cummins powered heavy towing truck where as Mercedes says to use 75w90.
I had a Dodge Ram with the G56 transmission. The consensus was that Mopar didn't want to add another part number, so they decided that ATF was good enough. A lot of us disagreed and sought a better choice.

Everyone should remember that SAE Auto oil weights are not the same scale as SAE Gear weights. 30W auto oil is about the same as 80W gear oil.
 
I often wondered about the manual transmission in the Legend. For lubrication, Honda specifies SE or SF, SAE 30, 10w-30, 10w-40 or 20w-40 engine oil.

I've been running Redline MTL for the last~150k miles.
 
MTL at 10.4cst starts out as a 30 grade and should be fine in you Acura Legend but it is on the thinner side. MT85 is also a 30 grade but a little thicker at 12cst which might be a better choice.

Honda MT fluid years ago was considerable thicker than it is now. It started out at ~11+cst 20+ years ago and now is ~7cst.
 
Shift quality for weak wristed consumers moved many to thin fluids

Cooling... improved fluid movement, fluid control, and component cooling....

Wear rate... the transmission still needs to last a certain amount of time

Usage.... expectations of consumer... ask the statisicians

And, MPG.... its just another fluid we can make thinner to save a few ounces of fuel every year without excessively wearing the transmission out before the warranty period expires

Toyota does use a 75w LV fluid in newer manuals that were designed for the lighter grade.
I’m not convinced they design anything for lighter fluids as much as lighter fluids end up being adequate based on whatever metrics they have or they design lighter fluids to provide adequate wear tendencies.



I do also want to ask this, anytime I change my transmission fluid with MT-90 the transmission shifts like butter. It’s such a phenomenal feeling. After 500 or so miles it goes back to normal. I doubt the additive packs is wearing out that quick. Maybe the oil it self is shearing a little bit, so what makes fresh MT90 shift so gooooooooood where as 1 month old MT90 doesn’t.
 
I am 100% sure that many did design manual transmissions for lighter fluids. They reduced hot spots drastically and improved fluid movement to where ever its needed.
 
I know this is more of an engineering question ...
For an industrial gearbox, assuming gear widths are correctly designed for the required loading, the normal criteria is the highest pitch-line velocity. There is an established lubrication table of that parameter v.s. oil viscosity.

My EV gearbox is a simple 2-stage reducer with the input running up to 10,000 rpm and uses a 70W GL-4 like Redline MT-LV, a choice which aligns with the table mentioned above. Most if not all other EVs use ATF which has nearly identical characteristics but is easier to source. If the oil was too thick for the PLV you'd get cavitation which could be damaging and is certainly audible as a whine.

Robotised manuals such as VW's DSG use a 70W or 70W-75 which allows brutally fast shifts. The gearbox design life is typically around 300,000 km from my understanding.

Conventional manual transmissions are pretty tolerant of varying oil viscosity because for any automotive application the actual service life is very short compared with industrial gearboxes. Just use what you like. I tried the entire range in my Suzuki SUV and found (not surprisingly) that shifting is generally best with thinner oils. I settled on Castrol VMX 80W based on not using it for towing and acceptable cold shifting.
 
For an industrial gearbox, assuming gear widths are correctly designed for the required loading, the normal criteria is the highest pitch-line velocity. There is an established lubrication table of that parameter v.s. oil viscosity.

My EV gearbox is a simple 2-stage reducer with the input running up to 10,000 rpm and uses a 70W GL-4 like Redline MT-LV, a choice which aligns with the table mentioned above. Most if not all other EVs use ATF which has nearly identical characteristics but is easier to source. If the oil was too thick for the PLV you'd get cavitation which could be damaging and is certainly audible as a whine.

Robotised manuals such as VW's DSG use a 70W or 70W-75 which allows brutally fast shifts. The gearbox design life is typically around 300,000 km from my understanding.

Conventional manual transmissions are pretty tolerant of varying oil viscosity because for any automotive application the actual service life is very short compared with industrial gearboxes. Just use what you like. I tried the entire range in my Suzuki SUV and found (not surprisingly) that shifting is generally best with thinner oils. I settled on Castrol VMX 80W based on not using it for towing and acceptable cold shifting.
This is amazing
 
I know this is more of an engineering question that most people won’t know the answer to but as the title suggests how do manufacturers go about selecting the oil weight they recommend for their manual transmissions? You have gear oil as thin as 75w all the way up to 75w90, and some even using ATF.

Why does one manufacturer go with 75w and the other 75w90. What makes 75w90 the fluid of choice in say a Toyota transmission, but not a 75w or a 75w80 fluid? And will using 75w really be that detrimental?...
The short answers are 1) horsepower throughput, 2) type of gear cuts and tooth finishes, 3) gear tooth coating, 4) Wear control and shifting effort testing with various viscosities and brands.
 
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What is the SAE weight of a typical Dex/Merc ATF?

Seems a lot of VW guys are using ATF in some of the older transaxles designed for 80W-90, GL4
 
What is the SAE weight of a typical Dex/Merc ATF?
The 'Grade' of the Dexron III/Merc was about a SAE 10W20, or a 100C viscosity of 7.5 cSt.

Seems a lot of VW guys are using ATF in some of the older transaxles designed for 80W-90, GL4
I wouldn't use a 7.5 cSt. fluid speced for a 15 cSt requirement.

It is the Kinematic Viscosity that matters.
 
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For an industrial gearbox, assuming gear widths are correctly designed for the required loading, the normal criteria is the highest pitch-line velocity. There is an established lubrication table of that parameter v.s. oil viscosity.

My EV gearbox is a simple 2-stage reducer with the input running up to 10,000 rpm and uses a 70W GL-4 like Redline MT-LV, a choice which aligns with the table mentioned above. Most if not all other EVs use ATF which has nearly identical characteristics but is easier to source. If the oil was too thick for the PLV you'd get cavitation which could be damaging and is certainly audible as a whine.

Robotised manuals such as VW's DSG use a 70W or 70W-75 which allows brutally fast shifts. The gearbox design life is typically around 300,000 km from my understanding.

Conventional manual transmissions are pretty tolerant of varying oil viscosity because for any automotive application the actual service life is very short compared with industrial gearboxes. Just use what you like. I tried the entire range in my Suzuki SUV and found (not surprisingly) that shifting is generally best with thinner oils. I settled on Castrol VMX 80W based on not using it for towing and acceptable cold shifting.
I like the attached screenprint which mainly pertains to a RWD transaxle type gearbox (VW air-cooled, specifically) because the hypoid gears and the synchromesh units share the same lubricant... but I believe there is merit in heeding the notion even in regular RWD transmissions and fwd transaxles (where, in both of which cases, the matter is not so acute).

Sorry re the small size of the screenprint.

Oh, one other comment: the "speed" gears are set on shafting and they freewheel. Some transmissions have them on two half-shell caged needle bearing arrangements. Others, like Mazda, actually run them as bushed/plain-bearing arrangements over top of, believe it or not, splines. The shafting for Mazda's sometimes then is drilled axially, the end of the shaft is equipped with an oil funnel/trough system, and those speed gears are radially lubricated between spline teeth, via this "pumped" shaft lubricant. My point in boring you all with this, is that varying from the mfr's suggested lubricant SAE viscosity is ill-advised, lest the efficiency of this lube scheme be diminished and premature wear occur.

Screenshot_20210807-100821.jpg
 
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Very interesting! As a recovering VW addict myself I've been into more than a few longitudinal transaxles. Subaru being the other common one. A few FWD and AWD VW/Audi still use this type of transaxle, where there is a ring&pinion as part of the transaxle. Whereas other transverse FWD transaxles don't have a direction change and the R&P are really just helical cut gears with a differential in the middle. This obviously increases efficiency which is why it's done.

Speaking to the Mazda gears riding on plain bearings on the mainshaft, many if not practically all older manuals were done like this. Having a hole or 2 drilled in the syncro to "pump" oil through the bearing and out the hole(s). This is why you can burn up a manual if it's flat towed in Neutral that doesn't have needle bearings. Without the counter shaft turning nothing gets any oil.

This is a late 70s and early 60s Ford Toploader side by side, I built recently. One is a 3 speed capable of holding 400+HP, and the other is a later overdrive out of a smog era Granada which may have made 150HP. Sorry, I don't have any pictures of any old VWs, that was long before camera phones.
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Very interesting! As a recovering VW addict myself I've been into more than a few longitudinal transaxles. Subaru being the other common one. A few FWD and AWD VW/Audi still use this type of transaxle, where there is a ring&pinion as part of the transaxle. Whereas other transverse FWD transaxles don't have a direction change and the R&P are really just helical cut gears with a differential in the middle. This obviously increases efficiency which is why it's done.

Speaking to the Mazda gears riding on plain bearings on the mainshaft, many if not practically all older manuals were done like this. Having a hole or 2 drilled in the syncro to "pump" oil through the bearing and out the hole(s). This is why you can burn up a manual if it's flat towed in Neutral that doesn't have needle bearings. Without the counter shaft turning nothing gets any oil.

This is a late 70s and early 60s Ford Toploader side by side, I built recently. One is a 3 speed capable of holding 400+HP, and the other is a later overdrive out of a smog era Granada which may have made 150HP. Sorry, I don't have any pictures of any old VWs, that was long before camera phones. View attachment 142575View attachment 142578View attachment 142576View attachment 142577
@Fabulous50s, 'Yo da man! That's cool! So few people pull them apart and rebuild them... so you, indeed, have a great understanding about the details. And I learned something today... re the flat towing prb with Mazda's 🙂.

That Ford TopLoader I believe was also fit, as an option and in some applications - with a BW OD unit. It was available I believe into the 70's. It was a very viable transmission with a bunch of novel features and behaviour... one of which was that as fit to a non-synchro first gear 'box, they would allow you to downshift into first with no gear clashing.

Non synchro first gear Toploaders were common. I imagine that they were still pretty drivable, but particularly with higher torque engines where you could tolerate 2nd gear even at low speeds, say, climbing hills.

I've always wanted to know, re non-synchro first gear transmissions... they are NOT like dog clutch non-synchro boxes that are meant to shift into low gear while still moving (of course), albeit by double-clutching... or are they? I don't think non-synchro first gear 'boxes are meant to be shifted into first by double-clutching...? Can they be? 'Dog-boxes, they have coarse dog-teeth and widish "slots" for those dog-teeth, and presumably some "lash" evident when the dogs are in their slots? Correct?

I'd really like to hear your- or anyone else's answers or comments re these questions!
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Yes the Borg Warner OD tailshaft unit has become very desirable in let last 10 years. The R10 unit was fitted to 6cyl cars, and us still quite plentiful. The R11 unit is the one everyone wants to put on their 4 speed and have a muscle car authenticity, but have an OD for at least teens for miles a gallon.

I opted for this red-headed-stepchild as I found it locally and am still able to retain my colum shift, which I love. Otherwise, I'd have gone with a typical M5OD or similar readily available 5 speed floor shift. But I wanted to keep the nostalgia of colum shift, since it was already in place.

As for the non-syncro 1st gear, @Cdn17Sport6MT. I'm not aware of Ford Toploaders that were non stncro 1st, they came out in 1964 when that would have been considered a bit archaic. It was Ford's answer to the 400+HP era, other transmissions were not up the task at the time.

I do own a '79 C-30 with the SM 465, it has the "granny low" which is not syncronized. It is basically a 3 speed with ultra low. This cannot be shifted into even with double clutching unless you are bairly moving. As it is something like a 6.5:1 ratio, and with 4.55 rear that is reaalllllllyyyyy slow. It is a dump truck with a worn out 350 so with a load of gravel or firewood it is very helpful, but man is it slow.
 
... My point in boring you all with this, is that varying from the mfr's suggested lubricant SAE viscosity is ill-advised, lest the efficiency of this lube scheme be diminished and premature wear occur.
Noting the thread topic, certainly there are going to be exceptions in unusual cases. But do you let that determine the rules for all cases? You won't see a design like that these days.
 
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Noting the thread topic, certainly there are going to be exceptions in unusual cases. But do you let that determine the rules for all cases? You won't see a design like that these days.
To this day, Mazda lubricates the "speed" gears (the free-rotating gears) with oil funnels, hollow shafts, radial holes, speed gears running on splines. They've done it for years. I don't know how common it is for other mfrs... but I believe the T5 Borg Warner / Tremec transmissions have oil funnels at least...
 
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