DEX VI insanity

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Hi all.Quick question about DexIII and DexVI, before you flame me for not searching I have and get mixed answers. My car is at 89,000 miles and I will be driving to Dallas this weekend. While I'm in Dallas I plan on changing the ATF. I would have liked to drop the pan and do a filter swap but logistics prevent me from doing so, plus I changed the filter at 60 K. Instead I will be using a mityvac to suck the fluid out from the trans fill tube and I already know that this method will only pull 40% out. I have already ordered 2 gallons of AC Delco DEX VI, and plan on doing the fluid exchange twice. Is this adequate? I'm well aware that Dex VI is backwards compatible with DexIII but have also heard that in some Allison transmissions DexVI can cause leaks. I'm also concerned that the remaining DexIII will cause issues. Am I overthinking this?car is a 2005 GTO mainly city driving with occasional high speed stints. TIA
 
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You can use Dexron VI with no problems
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Backwards compatible means you can mix any way at all with no problems. Just listen to GM. It will take many changes to get 99% D6. Always will be an amount of the D3 inside.
 
I don’t see the insanity to be honest. It is backwards compatible and they say so. It works well in my old BMW which calls for Dex III, and IMO if it works in that old thing I don’t know why it wouldn’t in other applications as well.
 
Dex VI is also full synthetic, so it can handle the heat a lot better. I actually use it in place of Mercon in my MGM's PS system, which has burnt fluid in the past, with no issues.
 
I used a Mityvac on a 2005 Park Ave. and got about 4 qts. out of the tranny, worked well. As others have said, you'll be in good shape with the DEX VI.
 
As they say, it is fully compatible. I have used Dex VI in the 4L60E in my 1996 Buick Roadmaster Wagon for well over a 100,000 miles now. It did several thousands of miles towing along with normal driving. No leaks, no issues. The trans now has 229,000 miles on it and shifts as good as it always has. I also changed over a 4T65E in my 2001 Grand Prix to Dex VI when it had 90k miles on it. I even did a UOA comparison and the Dev VI did better. No issues with that car either and I gave it away with 230k miles on it. My 4L80E in my Suburban has Dex VI in it now also, no issues.
 
Thanks to all for the great advice. My mityvac came in yesterday and the DEX 6
is supposed to be delivered on monday. I plan to take a few pics and maybe do a how to. I just wish it wasn't 100 degrees outside I'm no longer used to this heat. Cheers
 
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