Dex/merc 3 question

[SC Maintenance]

Ya see, DexronIII/Mercon starts out thicker but shears down more quickly to a lower cSt, and closer to what DexVI will start out and stay at.
DexronVI starts out thinner but stays in grade waaaaaaay loooooonger.

This is constantly repeated here - but how long does it take Dex3 that starts at 7.5 to shear to a Dex6 @ 5.8 level. Does Dex6 shear not at all - or will it be at 5.0 at some point?

How about more modern fluids that meet spec - like a fully synthetic Amsoil, that starts at 7.5? Is it going to shear immediately to 5.8?

 

[Pablo]

Ya see, DexronIII/Mercon starts out thicker but shears down more quickly to a lower cSt, and closer to what DexVI will start out and stay at.
DexronVI starts out thinner but stays in grade waaaaaaay loooooonger.

Even that is a generalization about base oil and VII types.

 

[Pablo]

This is constantly repeated here - but how long does it take Dex3 that starts at 7.5 to shear to a Dex6 @ 5.8 level. Does Dex6 shear not at all - or will it be at 5.0 at some point?

How about more modern fluids that meet spec - like a fully synthetic Amsoil, that starts at 7.5? Is it going to shear immediately to 5.8?

We just said the same thing haha

 

[Astro14]

This is constantly repeated here - but how long does it take Dex3 that starts at 7.5 to shear to a Dex6 @ 5.8 level. Does Dex6 shear not at all - or will it be at 5.0 at some point?

How about more modern fluids that meet spec - like a fully synthetic Amsoil, that starts at 7.5? Is it going to shear immediately to 5.8?

I think the bigger question - will your transmission care that the fluid is 5.8 or 7.5?

The presumption in the OP question is that there will be a performance difference between those two viscosities.

I don’t know that it’s true - I don’t think they are that sensitive. Folks have run older Aisin transmissions on WS, which has the lower viscosity, without issue.

So, if the transmission doesn’t care - why should the OP?

 

[Astro14]

Let me put it this way - the Dex III is already shearing across that range. The fluid he is using is already going from 7.5 to something lower - and the transmission hasn’t had a problem operating with fluid that varies across that range.

So, the evidence of the transmission operating correctly with fluid at, say, 5.8, is already there.

He just doesn’t want to pour in new fluid that is already at 5.8, even though the transmission has operated fine at that viscosity…

 

[SC Maintenance]

I think the bigger question - will your transmission care that the fluid is 5.8 or 7.5?

The presumption in the OP question is that there will be a performance difference between those two viscosities.

I don’t know that it’s true - I don’t think they are that sensitive. Folks have run older Aisin transmissions on WS, which has the lower viscosity, without issue.

So, if the transmission doesn’t care - why should the OP?

The answer is a giant "it depends"

My Toyota transmission shifts electronically and the torque converter locks in every gear. In that application the hydraulic requirements are comparatively minimal, so it makes sense to run the thinnest fluid that provides the needed lubrication - to help with crisp shifts and gas mileage.

Alternatively an old transmission with fewer gears and a locking converter turbine that is less efficient could be significantly affected by fluid viscosity. If it takes longer for your converter to lock the transmission will run hotter. You won't see the affect of that for tens of thousands of miles, but that does not mean it does not exist.

Can also be a big difference if I am taking leisurely drives in the country vs stop and go in Phoenix.

My point is, simply saying "Dex 6 is better" is not a completely true statement. Because ATF provides both a lubrication and hydraulic function, and the hydraulic side cares about viscosity.

If I had a AT designed in the age of Dex3 and specing Dex3 I would be running the Amsoil fluid at 7.5 cst.

 

[BlakeB]

I’ve used this in Dex III applications with good results. Any TES 295 or TES 668 fluid would serve you well, in my opinion. I use Supertech Mercon V in all of my Toyota power steering systems, which is also suitable for use in Dex III applications

 

[Kira]

both my old school Nissan transfer cases

Parameterize "old school", please. Model year range will do.
Seriously, I get old truck questions all the time.

 

[kschachn]

It’s not just improved viscosity retention, it’s also better oxidative stability and typically a better base stock (Amsoil and other premium fluids excepted).

 

[SC Maintenance]

Parameterize "old school", please. Model year range will do.
Seriously, I get old truck questions all the time.

Old school = gears and chains only that either lock both output shafts or not. No clutches or viscous coupling or other stuff that might require the fluid to be something other than purely a lubricant.

My 2 Nissans are in my sig but this type of case has been around since the late 40’s and still made if you buy right.

 

[SwampSurvivor]

DEXRON-III fluid is inferior in every aspect compared to DEXRON-VI. Base stock, viscosity retention and oxidation resistance being the three most important.

I've been runing Dexron-VI in my DIII spec'd transmission for years and it definitely likes the VI more than the III.

 

[Shucks18]

Let me put it this way - the Dex III is already shearing across that range. The fluid he is using is already going from 7.5 to something lower - and the transmission hasn’t had a problem operating with fluid that varies across that range.

So, the evidence of the transmission operating correctly with fluid at, say, 5.8, is already there.

He just doesn’t want to pour in new fluid that is already at 5.8, even though the transmission has operated fine at that viscosity…

“Even though the trans has operated fine at that viscosity” that’s a crazy assumption, all your doing is assuming my current or past trans fluid in my truck has sheared to the point of 5.8, you don’t know if it has or not. I do drain and fills all the time, so it likely has never dipped that low.

 

[Shucks18]

I’ve used this in Dex III applications with good results. Any TES 295 or TES 668 fluid would serve you well, in my opinion. I use Supertech Mercon V in all of my Toyota power steering systems, which is also suitable for use in Dex III applications View attachment 241666View attachment 241667
View attachment 241668View attachment 241669

Thanks for the response. I’ve always used valvoline dex merc, so I’ll probably stick with it. Maybe I’ll use this stuff in power steering systems. I most def wouldn’t use this stuff in my 94 ls400 that specs T IV tho, despite is says so