I have a 2012 Ford F-150 2WD 6 Cylinders Ecoboost 3.5L FI Turbo DOHC 213 CID with 150k miles, that I originally bought used, and I don't believe the transmission fluid has ever been changed. In previous videos you mentioned that you can change the fluid, keep the old fluid (in case that the transmission starts to slip) and add the old fluid back if it does. But I believe you also mentioned that not all of the fluid drains out and the new fluid mixes with the old. How might the mixing of the old and new fluid affect the transmission and should I even mess with it? I'm not having any transmission issues but I'm trying to figure out what I should do. Any suggestions would be appreciated.
From a fellow Texan.
For starters check the site FAQ.
There's no problem mixing the new fluid with old unless the transmission is worn to the point where friction material suspended in the old fluid is all that keeps it working. If you do 3 or 4 drain and fill procedures, driving around a while between times, that will replace most of the old fluid.
Changing fluid if the stuff in there really has not been changed in 150K miles is a risk. No one can predict with certainty what will happen. The reason to save the old fluid is in case the transmission slips or otherwise misbehaves when the new fluid is introduced. Be sure to replace the filter as well if applicable.
Transmission fluid changes should have been about every 50K miles. As @chucktobias indicated, at this point it's a risk. I would still rather change it out and hope for the best rather than knowing the trans will have to be replaced in the very near future if nothing is done.
