Wow... sitting down to read this thread in one sitting is like the blind man's elephant. An interesting study in group communication.
One guy had an 8.1 in a 2000-2006 GM truck that came with an Allison 1000, therefore he assumes that all 2000-2006 GM trucks that had the 8.1L also had the Allison 1000. So his good will efforts to help end up creating more confusion, that continues all the way to the last post of this thread (before mine) where another guy says the same thing, even though two other people already clarified that the 8.1L in the Suburban (and Avalanche and Yukon XL) didn't come with an Allison.
In fact, the OP's 2006 didn't come with a 4L80e either. It came with with a 4L85e. Yes, it is an upgrade based on the 4L80e, but it is a different transmission part number, and could, in theory, have a different fluid spec as part of the upgrade. So the best advice given was advice most often given... look at the manual first!
Both the 4L80e and the 4L85e use Dexron V1. Unlike mixing coolants using different chemistry, it won't hurt anything to mix different brands of Dexron VI, as long as the bottle clearly states that the fluid meets the Dexron VI specification. The coolant analogy of mixing different ELC's, HOAT's, etc doesn't really apply here. Dexron VI is a licensed fluid specification.
As the TSB explained, GM retired the former Dexron III specification. Any older bottles of Dexron III (or the popular dual specification Mercon/Dexron III, even if synthetic) should not be used in the 2006, as those bottles do not say Dexron VI. Oil formulators still produce transmission fluids that say "compatible with" or "suitable for"... but unless those bottles say in print "Dexron VI", then the forumlators didn't pay for the license.
If they didn't pay for the license, then their formulation didn't get tested to GM's standards. A decade ago, dual purpose fluid was common and ok. Not now.
"For example, the
maximum viscosity for Dexron-VI fluid
when new is 6.4 centiStokes at 100
degrees C, while the minimum newfluid
viscosity for Mercon V is 6.8 cSt
and often greater than 7.0 cSt. There is
no way a single fluid can meet both
specifications."
Brad Onofrio, Afton Chemical
To the OP... you are worried that the fluid you might add won't be compatible with the fluid the shop put in. What I would be more worried about is whether or not the fluid that the shop put in is compatible with your tranny.
The shop might have filled from a bulk container, and then "top treated" the fluid with an "additive" to make the fluid "compatible." This is common among lube shops, because it is too costly to maintain unknown quantities of 6 different automatic transmission fluid specs running around out there in recent vehicles, nevermind the legacy models.
"Using an older specification fluid plus a
top-treat to convert it to a new specification
can be seriously detrimental to fluid
performance, and is not recommended
by automakers. Performance that may be
compromised by using top-treated fluid
includes shifting, oxidative stability, fuel
efficiency and transmission life."
Brad Onofrio, Afton Chemical
So indeed, ask what they use, and have them show you where they pull it from. If it isn't licensed Dexron VI, then you might be better off dropping that fluid and getting the right stuff in there.
