I have a 2002 Z28 with 78,xxx miles with an automatic transmission that was purchased used. I wanted to drop the pan and change the tranny filter and fluid. Being I didn't know if there was Dextron 3 currently in the transmission or what I installed a new Purolator tranny filter and Dextron 6 ATF. I am wondering if the Dextron 6 is compatible with whatever is still in the tranny being that it wasn't totally flushed. Thank you.

 

GM says DEXRON-VI is OK, in fact preferred, for DEXRON III automatic (not manual) transmissions:

https://gm.oemdtc.com/TechLink/TechLink2009May.pdf

The above clearly indicates it can be used to top off previous versions of DEXRON in any proportion.

They did identify certain vehicles that are exceptions, including specific years of the Hummer, Saturn, Saab, and certain Chevy and Pontiac small SUV's:

http://www.revbase.com/BBBMotor/Tsb/...dPdf?id=168779