8l90 downshift characteristics - light throttle at 70mph

[2018GMC]

On 10/6/2022 at 6:20 PM, Miguel Olea said:

Update!  I've only driven the truck 5K miles since I had the service done.  My buddy and I dropped the pan changed the filter and did a flush with 20 quarts of the new blue label Mobil 1 fluid.  I also added 1 10oz bottle of the lubegard platinum to further protect against any shudder although I've never had that problem.  The truck drives and shifts so much better which leads me to believe that GM had THE WRONG fluid in that flush machine.  It probably had Dexron 6.

I'm thinking of doing a DIY at home flush on my 2018 6.2 with 8l90. Only has 20k on it and shifts great but have heard that the OEM fluid is hydroscopic and slowly absorbs water over time which causes shudder and shifting issues.

Looks like the Mobil 1 LV Blue label 124715 is $45 for a 6 pack on the jungle website.
I'm betting I need 12 quarts to do the flush at home, possibly more if i want to change transfer case atf as well which holds 3 pints.

After how many quarts did you notice the fluid coming out of the return line get cleaner?

Looks like I could do this at home DIY flush in the driveway by:
1. Drop pan, remove old fluid and clean pan, replace filter.
2. Install pan and add 4 quarts of fluid.
3. Remove the cooler block, inserting some poly line in the cooler port on side of transmission towards front of vehicle.
4. Fill transmission with 4 quarts of new fluid.
5. Run engine for 45 - 30 seconds to flush torque converter. Repeat until fluid is clean coming out of poly line.
6. Blow cooler lines out with compressed air
7. Replace cooler lines onto side of transmission.
8. Fill with new fluid and check level.

[Miguel Olea]

I really can't say when the fluid turned cleaner as mine only had 5k miles so it was still very clean.  I highly suspect GM used the wrong fluid (see above post) and that is why I changed it.  I used 20 quarts to make sure it was a complete flush.  Jegs had the best price with free shipping if I recall correctly.  I have the thermostat delete kit in mine so it was easy to flush.  One hose barb in the return port and I was good to go.  I also bought an adaptor that screws into the check fill port of the pan and I was able to use a drill pump to pump the new fluid in.  Shifts improved immediately after the flush.  I had my doubts when doing the flush, but I guess GM did have some other fluid in that "flo tech" flush machine.