I am going to disagree with some of the recommendations above and I'll tell you why, in detail.
I am a big fan of DEX VI ... most of the time. But not in your application. I'll explain, to the best of my memory and ability to recall details.
Dex VI is a wonderful product enhancement "next step" past Dex III(h); it came along around 2006. DexVI has performance requirements which essentially necessitate the use of base stocks which would be grp III or greater, and the additive package is very robust. It's a great product, unless it's paired with older transmissions which have older seals and o-rings which are not up to the capability expectations for the new fluid. GM back-spec'd the Dex VI because they were not going to continue to license the Dex III. However, Allison did not agree, and created a "new" spec just so they could continue to have a separate fluid for the "old" seals. (What was known then as TES-389, essentially a traditional Dex III; versus their TES-295 which was a PAO based Dex III). Newer transmissions which don't have the seal compatibility issues can benefit greatly from the Dex VI, as it maintains its viscosity incredibly well (though it is lower; around 6.0 or so), and has a fantastic additive package.
Way back in the day, I did a lot of research because my (then) new 2006 Silv 3500 had the Dmax and 6-spd Allison (which was GM made in Baltimore). Whereas GM said it was proper to put Dex VI in the trans, Allison didn't agree and spec'd TES-389 or TES-295. GM owned Allison at the time, so there was some internal in-fighting over the topic. There are a few old-time members here who actually worked at GM and Allison; I was able to get the backstory from them. They greatly respected each other, but had differing opinions on the issue. The issue lies in the fact that when DEX VI was used in very high-heat conditions, the fluid would hold up fine, but if that application included old seals, there could be a breakdown of the seal materials. This was a VERY rare circumstance, as the very high heat took a long time to degrade the materials when combined with the additive package compounds. But, the condition does exist, regardless of how seldom the actual occurrences are. In "normal" use, it's unlikely that the heat would ever get high enough to cause a problem.
Since 2007, the seals in GM vehicles have been compatible with Dex VI. 2006 was a crossover year; some had upgraded seals and some didn't. Any vehicle older than 2006 won't have the updated seals. And as long as the fluid doesn't get really hot (above 300F IIRC), it "should" be OK for Dex VI. But that's not a risk worth taking IMO because there are plenty of alternatives which don't have this risk, no matter how remote it may be. So, with all that info, and knowing I was going to be towing an RV and had potential for high heat conditions, I decided to eschew the Dex VI and use a TES-295 type product. My truck didn't make the serial number cut-off, so it was susceptible, however remote that chance might be.
OP, if you want to service your 2002 truck, I would recommend a few choices:
- any well-known brand of lube that offers a "Dex/Merc", "D/M", "D3-M" or such fluid; plenty to choose from. These will be traditional grp II/II+ type fluids. The will start with vis around 7.3cSt or so, but fall off quickly. However, the vis drop doesn't seem to affect wear control; I've seen plenty of UOA to support this conclusion.
- any quality brand which offers a synthetic version comparable to the old Dex/Merc applications. Again, there are plenty to choose from. Some will be group III, others PAO. Any will suffice and offer a longer service life due to the higher base stocks.
That's my input on the topic.
