GL-4/GL-5 gear oils a compromise?

Originally Posted by Brian553
I'm unaware of any GL-5 MTFs. Motul gear 300 is edging on saying it's designed for MTs with "For synchronised or not synchronised gearboxes..." but that isn't communication to me that it has the correct friction modification to assist (/for) synchros.

Subaru used to make a GL-5 MTF, but their newest fluid is believed to just be "plain" GL-5.

Subaru manual transmission owners are still hungry. Mola, we need your help.



Why would you think that when Motul calls the Gear 300 a "transmission lubricant" and states that it is "for synchronised or not synchronised gearboxes" that this would be false? Unless you're believing the "GL-5 is for differentials" nonsense that seems to be out there.

Did you read my quotes from MolaKule above?

And let me state this for the nth time, modern GL5 lubricant formulations are safe in most components/units. When there is a mismatch, it is usually a mismatch between viscosity and friction modification."

I would think that if he no longer believed this statement he would say so and offer science to back up why. But if anything the quality of synthetic fluids are even better now that when he wrote that.

It's anecdotal but I know a lot of Subaru STI owners who have successfully used the Motul Gear 300 for years.
 
Last edited:
You are right; I missed that they specifically call it out as a transmission lubricant on their site. That gives me the confidence that it's designed to assist with synchros.

I already understand that GL-5 will provide enough EP and AW protection for MTs.
 
A MT manufacturer has the option to select what EP additive will work the best in it's transmission if they have their own MT oil product. Also the amount installed in the oil.

What gets me about Mazda's requirement was if the OEM 75w-90 oil was not available then you can use that weight in a GL-4 class. So from what I know is GL-4 has less EP additive than GL-5 and even though a oil says that it does not corrode yellow metals, still the GL-5 is not what you want for a transmission. (Although the EP additive is buffered)

Now the ball thrown by the aftermarket oil company has no particular transmission in mind.
 
Originally Posted by Bill_W
A MT manufacturer has the option to select what EP additive will work the best in it's transmission if they have their own MT oil product. Also the amount installed in the oil.

What gets me about Mazda's requirement was if the OEM 75w-90 oil was not available then you can use that weight in a GL-4 class. So from what I know is GL-4 has less EP additive than GL-5 and even though a oil says that it does not corrode yellow metals, still the GL-5 is not what you want for a transmission. (Although the EP additive is buffered)

Now the ball thrown by the aftermarket oil company has no particular transmission in mind.


Did you ever consider that maybe the "aftermarket oil company" knows what they are doing? If they didn't, don't you think every Internet forum out there, including this one, would have blown up with stories of broken transmissions? The sales gained from lying to people wouldn't be worth it.

I now know that you will NEVER accept anything beyond your point of view and don't actually want to learn despite you saying otherwise.

"GL-5 is not what you want for a transmission" is a myth and I quoted both Bob and MolaKule on this site above saying as much.

As I've said before, I understand if you want to preserve your warranty then you'd want use a straight GL-4. Although if I have an issue with the transmission using the Motul Gear 300 that is rated GL-4/GL-5 then I'm going to fight it every step.

But PLEASE stop spreading misinformation by saying that all GL-5 fluids are equal and only suitable for differentials.
 
Last edited:
Bottom line if a manufacturer specifies GL-4 fluid...USE ONLY GL-4 fluid, not GL-4/5, GL-4+,ect

GL-4 is not actually "obsolete" because some manufactures STILL specify it for new cars sold today. Even if API says it is.

A MAJOR issue using some fluids that indicate GL-4/5, GL-4+ is that they will cause balky shifting in many models of cars with synchronized manual transmissions.
 
Originally Posted by AC1DD
Bottom line if a manufacturer specifies GL-4 fluid...USE ONLY GL-4 fluid, not GL-4/5, GL-4+,ect

GL-4 is not actually "obsolete" because some manufactures STILL specify it for new cars sold today. Even if API says it is.

A MAJOR issue using some fluids that indicate GL-4/5, GL-4+ is that they will cause balky shifting in many models of cars with synchronized manual transmissions.


So you believe that modern synthetic oils that are rated GL-4/GL-5 are lying to us all?

I, and others, are successfully using the Motul Gear 300 in GL-4 applications with zero issues other than a slightly harder 1-2 shift when the transmission is cold due to cold ambient temps. Less than 10 degrees Celsius. The problem goes away after 2-3 minutes of driving. I certainly don't think it's damaging anything.

I had it in my last MX-5 in 120K with using When I drained the OEM fluid it looked awful after 25K. When I changed the Gear 300 after 75K it looked as good coming out as the new fluid.
 
Last edited:
Originally Posted by sjd
Originally Posted by AC1DD
Bottom line if a manufacturer specifies GL-4 fluid...USE ONLY GL-4 fluid, not GL-4/5, GL-4+,ect

GL-4 is not actually "obsolete" because some manufactures STILL specify it for new cars sold today. Even if API says it is.

A MAJOR issue using some fluids that indicate GL-4/5, GL-4+ is that they will cause balky shifting in many models of cars with synchronized manual transmissions.


So you believe that modern synthetic oils that are rated GL-4/GL-5 are lying to us all?

I, and others, are successfully using the Motul Gear 300 in GL-4 applications with zero issues other than a slightly harder 1-2 shift when the transmission is cold due to cold ambient temps. Less than 10 degrees Celsius. The problem goes away after 2-3 minutes of driving. I certainly don't think it's damaging anything.

I had it in my last MX-5 in 120K with using When I drained the OEM fluid it looked awful after 25K. When I changed the Gear 300 after 75K it looked as good coming out as the new fluid.


I'm suggesting that the car manufacturer knows their vehicle best and if their engineers recommend something there is a good reason for it. When the warranty is over do as you please but if you have a warranty in effect use the recommended specifications.
 
Originally Posted by AC1DD
Bottom line if a manufacturer specifies GL-4 fluid...USE ONLY GL-4 fluid, not GL-4/5, GL-4+,ect

GL-4 is not actually "obsolete" because some manufactures STILL specify it for new cars sold today. Even if API says it is.

A MAJOR issue using some fluids that indicate GL-4/5, GL-4+ is that they will cause balky shifting in many models of cars with synchronized manual transmissions.


It is a fact that most fluids designed for purely GL-5 Hypoid differential applications have, on average, a higher viscosity than dedicated GL-4 rated MTF fluids of the same SAE grade.

For example, a 75W90 GL-5 Hypoid differential fluid will have a viscosity at least 3-5 cSt higher than a 75W90 dedicated MTF GL-4 fluid.

Sometimes even a slight viscosity increase of 2-3 cSt can cause extra shifting effort in cold temps.
 
Last edited:
Originally Posted by AC1DD
I'm suggesting that the car manufacturer knows their vehicle best and if their engineers recommend something there is a good reason for it. When the warranty is over do as you please but if you have a warranty in effect use the recommended specifications.


The car manufacturers have to make compromises and so do their engineers, usually dictated by cost versus any engineering reason.

However, you do make a valid point that if you want to ensure no warranty issues then yes, obviously play it safe and do what the factory recommends.

I see things differently and think there are a lot of areas where the performance/motorsports aftermarket is superior to the factory. One of these areas is fluids.

I should add that I am not affiliated with Motul in any way. I've just been a very satisfied customer for 14 years with a host of different vehicles.
 
Originally Posted by sjd
Originally Posted by AC1DD
I'm suggesting that the car manufacturer knows their vehicle best and if their engineers recommend something there is a good reason for it. When the warranty is over do as you please but if you have a warranty in effect use the recommended specifications.


The car manufacturers have to make compromises and so do their engineers, usually dictated by cost versus any engineering reason.

However, you do make a valid point that if you want to ensure no warranty issues then yes, obviously play it safe and do what the factory recommends.

I see things differently and think there are a lot of areas where the performance/motorsports aftermarket is superior to the factory. One of these areas is fluids.

I should add that I am not affiliated with Motul in any way. I've just been a very satisfied customer for 14 years with a host of different vehicles.

+1
Whilst car manufacturers are basically engineer-centric in the early days, but the same cannot be said in modern days among others on lubricants-related recommendations by OEM .
Compromises are typically well considered by engineers , but corporate accountants ..............................
 
Last edited:
Originally Posted by AC1DD
Originally Posted by sjd
Originally Posted by AC1DD
Bottom line if a manufacturer specifies GL-4 fluid...USE ONLY GL-4 fluid, not GL-4/5, GL-4+,ect

GL-4 is not actually "obsolete" because some manufactures STILL specify it for new cars sold today. Even if API says it is.

A MAJOR issue using some fluids that indicate GL-4/5, GL-4+ is that they will cause balky shifting in many models of cars with synchronized manual transmissions.


So you believe that modern synthetic oils that are rated GL-4/GL-5 are lying to us all?

I, and others, are successfully using the Motul Gear 300 in GL-4 applications with zero issues other than a slightly harder 1-2 shift when the transmission is cold due to cold ambient temps. Less than 10 degrees Celsius. The problem goes away after 2-3 minutes of driving. I certainly don't think it's damaging anything.

I had it in my last MX-5 in 120K with using When I drained the OEM fluid it looked awful after 25K. When I changed the Gear 300 after 75K it looked as good coming out as the new fluid.



I'm suggesting that the car manufacturer knows their vehicle best and if their engineers recommend something there is a good reason for it. When the warranty is over do as you please but if you have a warranty in effect use the recommended specifications.


+
I've went by the same definition on vehicles since I've been driving. OEM or 'approved' options on most things. The GL-4 rings a bell. Think I went around the search on that one with an older VW and the soft metal innards.
I agree there are many good fluids out there and sure to be some that are spec'd and perform well above the mfg standards or 'warranty needs' if one wants to dabble or is beyond warranty.
I think Redline was a strong recommendation on a clutch job I had done for an older 6MT.

* Most of this stuff can be easily referenced or researched by aficionados on car forums specific to brands, models, years or certain variations.
smile.gif
 
Last edited:
I have heard good things about Redline products and Amsoil too. The Motorcraft XT-M5-QS is popular with Miata NC owners (previous model) I see Castrol makes it for Motorcraft. Of the list that MolaKule made I stopped at #1 on the list and going to try Amsoil MTF.
 
Originally Posted by Bill_W
I have heard good things about Redline products and Amsoil too. The Motorcraft XT-M5-QS is popular with Miata NC owners (previous model) I see Castrol makes it for Motorcraft. Of the list that MolaKule made I stopped at #1 on the list and going to try Amsoil MTF.


Amsoil MTF is a good product!
 
Originally Posted by Bill_W
Wondered if this view can help?

https://www.widman.biz/uploads/Transaxle_oil.pdf


This really explains it all, I think with enough detail.

But the simple explanation is that GL-5 exceeds GL-4 Extreme pressure limits, by having about twice the Sulfur/Phosphorous EP additive. But nowhere in the GL specs do they mention synchronizers, and the level of sulfur/phos additives in GL-5 causes extra wear of synchronizers. You can claim GL-4/GL-5 on any GL-5 formulation.

The GL-4+ formulations on the market today are formulated with Calcium and/or Magnesium sulfonates that give you better than GL-4 EP performance and are not only less damaging to synchronizers, but help them grab and wear less. They can be used in any GL-4 application. And most of the manual transmissions of the last 10 years have been specifying this (although normally by there own trade names.
 
Mobil Delvac 1 Transmission Fluid 50 has a slightly higher Cst at 40 C....

132 verses around 100...

But they claim you can use it as GL-4 gear oil
Yes, I actually recommend TO-4 SAE 50 oils for countries where there is no GL-4 specific oil in that viscosity range (mentioned in my paper). it is one way to make sure you don't have any sulfur/phos additives. And if it can handle the power train of a D10, a car is a piece of cake.
 
But the simple explanation is that GL-5 exceeds GL-4 Extreme pressure limits, by having about twice the Sulfur/Phosphorous EP additive. But nowhere in the GL specs do they mention synchronizers, and the level of sulfur/phos additives in GL-5 causes extra wear of synchronizers. You can claim GL-4/GL-5 on any GL-5 formulation

Not quite. Today dedicated MTF's with GL-4 protection ratings contain anti-wear additives, not EP additives. The GL specifications never dictate the additive package or chemistries to be used, because they define wear protection levels for various transmissions and gear types.

The GL-4+ formulations on the market today are formulated with Calcium and/or Magnesium sulfonates that give you better than GL-4 EP performance and are not only less damaging to synchronizers, but help them grab and wear less. They can be used in any GL-4 application. And most of the manual transmissions of the last 10 years have been specifying this (although normally by there own trade names.

The Calcium and/or Magnesium components do not define the wear protection, rather they are part of the friction modification chemistry group.
 
Last edited:
Been shopping for an alternative to the OEM 75w-90 GL-4 gear oil for my 2019 Miata MT. Aftermarket gear oil has some GL-4 and GL-5 combo in that weight. Am I better off looking at just the GL-4 75w-90 gear oil?

The answer is yes, look for a dedicated 75W90 MTF oil with a GL-4 rating such as Amsoil's MTG or Redline's MT-90.
 

Carquest said:
PRODUCT DETAILS
Part No. 990-01
Warranty Details (30 DAY REPLACEMENT IF DEFECTIVE)
CARQUEST® EP 90 GL-4 Gear Oil is mineral based extreme pressure multi-purpose gear oil containing antioxidant, anti-corrosion and anti-foam additives. It is designed to protect gear boxes, mildly loaded axles and transfer boxes. It contains extreme pressure additives to provide protection against the high rubbing pressures experienced in hypoid units. CARQUEST EP 90 GL-4 Gear Oil is primarily recommended for the lubrication of spur and helical gears, such as those used in automotive manual gearboxes, where the manufacturer specifies the use of EP 90 gear oil.

Product Features:
  • Extreme Pressure Performance
  • Protects Hypoid Gears
  • Excellent anti-wear properties
  • Anti-corrosion and anti-rust properties
  • Recommended for GL-4 applications
This is not an appropriate oil because 1. it is a straight 90 grade, 2. EP additives are only needed in Hypoid differentials requiring an EP additive for GL-5 protection, 3. It does not contain the specialized friction modifiers needed for synchronizer assemblies.
 
Back
Top