If your car has a lot of miles and you are not sure about trans fluid service, DO NOT DO A FLUSH. At most change 4 quarts at each of the next 3-4 oil changes. Your trans has become used to wear materials and uses them for friction in the clutch packs. A sudden change of all the fluid can result in slipping. This means time for a new trans. The slow change in fluid should prevent the slipping.

 

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