Older Mobil1 ATF in Dex 6 spec'd auto?

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Just curious for some opinions. I was going to do a pan drain/refill in the big 6L90E trans crammed into my CTS-V. She has 41,000 miles on her. I have a ton of the pre-Dex6 Mobil 1 ATF I'd like to use up. As far as I can tell a drain and fill will net me about a 50% replenishment.

Pretty easy to say "oh it specs this only" The most reputable performance rebuilder of the sister transmission, the 6L80E, uses Dex III in their rebuilds because they see no benefit from Dex6, only added cost and lower viscosity.
 
Dex 6 is designed to run longer and have more stable viscosity throughout its life. It is thinner new, but Dex IIIH seems to sheer to that and lower levels fairly quickly.

I think you'd be okay to run Dex III in it, but personally I would spend the extra. I would also wonder if the line pressure values in the PCM are designed around having a thinner fluid that is more consistent as well. It would probably self adjust but I know from tuning they are arbitrary numbers, not PSI values at least in the 4L80/60s I've seen.

Food for thought.
 
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Seems unlikely to cause an issue. The M1 eventually shears down to a Dex VI viscosity. See link for one of my older UOA comparing M1 ATF IIIh compared to Dex VI in the same transmission. My preference is to always use the correct fluid, but I don't see it would be very likely to cause an issue. The below UOA also validates that the thinner Dex VI works just as well or better than the thicker M1 ATF.

UOA comparison
 
Originally Posted By: Gasbuggy
I have a ton of the pre-Dex6 Mobil 1 ATF I'd like to use up. As far as I can tell a drain and fill will net me about a 50% replenishment.

The most reputable performance rebuilder of the sister transmission, the 6L80E, uses Dex III in their rebuilds because they see no benefit from Dex6, only added cost and lower viscosity.


I don't know who this builder is but they are highly misinformed.


Recommend you go with Dexron VI and sell the old Mobil stuff to the performance rebuilder.
 
Originally Posted By: PolarisX
Dex 6 is designed to run longer and have more stable viscosity throughout its life. It is thinner new, but Dex IIIH seems to sheer to that and lower levels fairly quickly.

I think you'd be okay to run Dex III in it, but personally I would spend the extra. I would also wonder if the line pressure values in the PCM are designed around having a thinner fluid that is more consistent as well. It would probably self-adjust but I know from tuning they are arbitrary numbers, not PSI values at least in the 4L80/60s I've seen.

Food for thought.




Interesting thought. This trans will be getting tuned in the next month or two. On the CTS-V forum it is a well known issue that most transmissions are either a quart low or need an extra quart. Owners run into issues where the engine is catching the rev limiter right before a wide open throttle shift. It's happened to me a few times. Adding 1 quart seems to solve it for everyone.
 
Originally Posted By: MolaKule
Originally Posted By: Gasbuggy
I have a ton of the pre-Dex6 Mobil 1 ATF I'd like to use up. As far as I can tell a drain and fill will net me about a 50% replenishment.

The most reputable performance rebuilder of the sister transmission, the 6L80E, uses Dex III in their rebuilds because they see no benefit from Dex6, only added cost and lower viscosity.


I don't know who this builder is but they are highly misinformed.


Recommend you go with Dexron VI and sell the old Mobil stuff to the performance rebuilder.




I'm sure they have a skewed point of view and are not considering long drain interval protection. I will have to locate the thread I read. Their opinion was formed more along the lines of how one vs the other lasted under track abuse between rebuilds for clients.
 
Originally Posted By: WishIhadatruck
Seems unlikely to cause an issue. The M1 eventually shears down to a Dex VI viscosity. See link for one of my older UOA comparing M1 ATF IIIh compared to Dex VI in the same transmission. My preference is to always use the correct fluid, but I don't see it would be very likely to cause an issue. The below UOA also validates that the thinner Dex VI works just as well or better than the thicker M1 ATF.

UOA comparison



Very interesting UOA. Perhaps I will just use Dex6 or RedLine D6 and sell or hang onto the M1.
 
I wouldn't use it on a CTS-V! Not an expensive high performance car like that! The Dexron VI is more shear-stable. The price has also come down considerably; even Walmart has a licensed Dexron VI for $4-something. Maxlife isn't licensed Dexron VI, but it's very popular on here.

On a Cobalt or Malibu, I'd use the old M1 for short drains, but not on a CTS-V! Save the M1 for the Accord, Mercedes, and LR in your sig
smile.gif


If you like name brands, Castrol Full Synthetic ATF is on sale at AAP/CQ 2 for $11.

If your transmission doesn't have a drain plug, get the Dorman pan that includes one
smile.gif
 
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