Heard a guy at the parts counter say you should never flush a transmission with over 100k miles

I was grabbing a filter at the NAPA in Springfield yesterday and overheard a customer telling the clerk that rule like it was gospel. It got me thinking, because I've done a bunch of those flushes on high-mileage cars in our shop and never had one fail after. The fear is that new fluid will loosen up old clutch material and cause slipping, right? But if the transmission is already on its way out, the flush just speeds up the inevitable. I've found a full machine flush with the right fluid, like Valvoline MaxLife, actually helps more often than not. It cleans the valve body and cooler lines, which can fix weird shift issues. Maybe the old rule needs a second look. What's your shop's take on high-mileage transmission service?

3 comments

3 Comments

mila_murphy211mo ago

Oh man, I am SO with you on this. That old rule is just fear talking. We do them all the time at our place too, and honestly, a proper flush with good fluid has saved way more transmissions than it has hurt. You hit the nail on the head, if it's already dying, new fluid just shows you the truth. But most of the time, the fresh fluid cleans out all the gunk in the solenoids and the tiny passages, and the thing just starts working RIGHT again. It's like giving it a fresh start.

willowh201mo ago

Tell me about it, that old myth needs to die already.

faith_thomas1mo ago

Exactly. I had a truck shifting rough for months, did a full flush, and it smoothed right out. The old fluid was just full of clutch material and varnish. Never had a problem again for years.