2009 Subaru Forester — one real-world flush test on a high‑mileage transmission

The speaker put a commonly stated mechanic/internet claim to the test: that flushing a high‑mileage transmission with dark, burnt fluid will 'destroy' it. The subject was a 2009 Subaru Forester with shifting problems described as 'shuttering' primarily in second and third gear and fluid that smelled bad and appeared dark. The pan magnet and filter showed metallic debris and a coppery, sparkly residue; the vehicle was stated as having 170,263 mile (later noted as 178,000 miles) and no prior service on the transmission.

Procedure and measured inputs: the speaker drained the pan, removed a mesh screen filter, inspected the valve body (reported as looking normal), installed a new filter, and used a flush machine while adding Max Life multi vehicle ATF (sold as compatible with Subaru). He primed with two quarts in the pan and planned to push about 10 quarts through the cooler (the video later references 12 quarts having been cycled). After the flush, the dipstick fluid was bright red; the speaker drove the car in mixed city/highway conditions and reported no immediate slipping. At ~16 miles of driving and then at 137.6 miles of cumulative driving after the service, the previous 'shuttering' and harsh upshifts were gone and the transmission felt smooth. The dipstick fluid darkened again over that mileage, which the speaker attributed to long-term buildup that a single flush cannot completely remove.

Conclusion and caution: in this single test the flush + new filter + compatible ATF improved shift quality without causing catastrophic failure. The speaker explicitly contrasts that result with common warnings from the internet and one shop that said 'fresh fluid won't fix' an already-damaged transmission; another shop offered to perform a flush. He notes that if a transmission already has complete slipping or no engagement, a flush will not help and replacing internal parts (or a reman unit, quoted as about $4,000) may be required. Final stance: a flush can help in some high‑mileage cases with symptoms like 'shuttering', harsh shifts, or delayed shifts, but it remains a gamble and longer-term monitoring is necessary.