Dexron VI
Use in 4L60E, 4L65E, 4L70E, 4L80E, 4L85E, 6L80 (pre-2015), 700R4 (modern refill). Most modern GM RWD 4-speeds.
When to use: Stock spec for nearly all GM 4-speed and early 6-speed RWD.
Wrong-fluid consequence: Wrong-fluid use is rare since Dexron VI replaced Dexron III. Mostly safe to use as a Dexron III replacement.
Dexron HP
Use in 8L90, 8L45, 10L80, 10L1000, and 6L80 (2015+ updated spec). Modern GM 8-speed and 10-speed.
When to use: Required for the friction-sensitive newer GM generations. Mandatory in 8L90 - using Dexron VI causes IMMEDIATE shudder.
Wrong-fluid consequence: Putting Dexron VI in an 8L90 = slip + shudder within 100 miles. Drain twice with Dexron HP to recover. Wrong fluid is the #1 cause of 8L90 TCC complaints.
Check your owner's manual or the dipstick stamp before pulling fluid. The 8L90 specifically is unforgiving - wrong fluid voids the warranty and the rebuild.