car myths last forever, apparently

Between Facebook and a non-technical employee at my (formerly?) favorite auto mechanic’s shop, I encountered two myths related to car repair I feel MUST be busted right here and right now.

1) Changing your transmission fluid will break or damage your transmission.
2) Brake calipers must be replaced in pairs.

Let’s take that second one first.


Brake disc and caliper photo from

This myth, like many of those which plague motorcycle riders, is based in a car repair truth made obsolete by modern technology. Back in the 1950s and ’60s and even in the 1970s and probably the ’80s to a large extent, yes, you did need to replace your brake calipers in pairs most of the time. If one went bad, chances are the other one wasn’t far behind it, but more importantly, the braking system internals worked at their absolute best when the calipers operated at the same level. When your safety in 1962 relies on the performance of your brake calipers, you replace them in pairs.


People … it’s 2019.  Braking systems are so advanced now they could probably control a moon landing. ABS systems are incredibly technologically sophisticated, and they adapt to the condition of the calipers on the fly, making hundreds (if not thousands) of adjustments a second as you mash the brake pedal in a panic stop.

In other words, your modern car can handle mismatched caliper operation better than you can possibly imagine. Replace the one broken caliper and get on with your life. The caliper opposite the broken one should be inspected for smooth operation, cleaned and adjusted if necessary, and if you’re really concerned about how your brakes function, flush, fill and bleed the entire braking system from stem to stern. Frankly, if you’d done that once a year anyway, your one bad caliper probably wouldn’t have failed in the first place.

On to the transmission fluid myth.


Transmission illustration from

I went to my auto mechanic for an oil change this morning, and while I was there I queried the clerk at the desk about having my transmission fluid changed at my next oil change interval.

“Oh, you shouldn’t do that,” they said. “That will break your transmission.”

I want to point out that the person who said this to me was not a mechanic, but an office worker. I’m sure they’ve been exposed to a lot of information in the time they’ve been working there, but this is one myth that needs busted immediately, if not sooner.

Replacing the oil in your transmission – and that’s what “transmission fluid” is, oil – DOES NOT BREAK OR DAMAGE YOUR TRANSMISSION IN ANY WAY. PERIOD.

What damages your automatic transmission is the tens of thousands of miles you’ve driven it without having the transmission serviced. Oil in any closed system is supposed to do two things: lubricate moving parts and cool those same moving parts. That’s it. That’s oil’s whole job in the context of your motor vehicle.

You change the oil (and filter) for your engine every 5,000 or 10,000 miles (PS stop changing it every 3,000 miles – that frequency is another old practice that was once necessary but is no longer and has become a widely propagated myth). You do this to ensure long life and peak performance from your engine.

Why wouldn’t you do the same thing for your transmission?

If your vehicle’s manufacturer doesn’t publish a recommended transmission oil change interval, take it upon yourself to have your transmission serviced every 15,000 or 20,000 miles. Just do it. Your transmission will thank you. For what it’s worth, I change the transmission oil in my dry-clutch BMW motorcycles every 10,000 miles.

(PS There are two ways to change your transmission oil – a simple drain & fill and a more thorough flush & fill. The drain & fill only replaces about half of the oil, because the rest is inside the torque converter. If you do this, you’ll have to do two or three drain & fills to get most of the oil swapped for fresh. A flush & fill uses specialized tools mechanics have to force new fluid in and old fluid out of every nook and cranny, including the torque converter. Costs more as a one-time thing, but probably the same as doing 2-3 drain & fills, so you come out even if you’re doing it right.)

The reason the myth “changing your transmission oil breaks your transmission” persists is because people don’t change their transmission oil. They leave it in there for 100,000 miles, then panic and change it. The thing is … the transmission is already damaged, you just don’t know it yet.  Imagine what your engine would look like inside if you left the same oil in there for 100,000 miles, how damaged it would be.

Oils used in most passenger vehicles have detergents and other additives in it that, when fresh, will literally clean out the inside of your engine (or transmission). The filter catches much of those substances, and with regular changes, those contaminants are removed from the system. Only problem is you haven’t been changing your transmission oil – let alone the transmission oil filter.

That’s right – there’s a filter in there. It’s probably been clogged with goo for 30,000 miles or more, so the goo goes elsewhere in the transmission, sitting there all sludgy until you finally have your transmission serviced… which is when that sludge is broken loose and becomes free to clog or stick to parts of the transmission it’s not supposed to – and thus adversely affecting the performance of your transmission, which you then think is broken. The truth is, your transmission was going to break sooner or later. Changing the fluid just tipped the scale towards sooner – and it’s your fault, because you never had it changed!

(Here’s a skippable explanation of why: As the transmission fluid breaks down, it loses its lubrication capabilities. As that happens, the metal parts in the transmission wear, releasing bits of metal, which stay suspended in the fluid. The discs in the clutch pack will actually use those bits of metal to assist friction – so when you take away the contaminated fluid that’s been helping the clutch discs stick together and replace it with clean, fresh fluid that lubricates properly, yeah, your transmission will slip.)


Photo of Honda Element used under IFCAR [public domain] via Wikimedia Commons.

(I didn’t have time or the inclination, frankly, to tell the shop clerk my little van (Honda Element) has a manual transmission, which means it contains gear oil, not standard transmission fluid – and there’s no filter, either. Gear oil is different than transmission oil – has a different purpose, different composition, etc.)

If you don’t want your transmission to die simply because you have the fluid changed, then take care of your transmission just like you take care of your engine. For me, I change the engine oil every 5,000 miles. The automatic transmissions in my household get their oil changed every 20,000 miles, or every fourth oil change (unless the manufacturer has a specified interval). I do this primarily because I almost exclusively buy used cars, and chances are the previous owner didn’t have the transmission serviced regularly; it’s up to me to try to stave off failure by keeping the transmission running at peak efficiency.

(My manual transmissions get fresh fluid every 50,000 miles (Honda’s spec is 90,000 for “normal driving” and 30,000 for vehicles operated in harsh conditions or used for towing. I tow motorcycles on a somewhat regular basis and often sit in traffic, so I hedge towards the short end of their timeline.)

Don’t buy into the hype of the “sealed transmission” many manufacturers tout these days. “Lifetime fluid” is no such thing if you plan to drive a car for several hundred thousand miles. Sealed transmissions should have their fluid changed at 80,000 or 100,000 miles – granted, that is the lifetime of some vehicles.

Anyway, changing the fluid causing your transmission to break is a myth. Don’t believe it. Have your transmission serviced regularly and you won’t have any reason to buy into this particular myth.

(For a visual explanation – complete with a disassembled transmission – of this issue and a reinforcement of the mythological status of “changing the transmission fluid broke my transmission,” check out Chris Fix’s video “Can Changing your Transmission Fluid Cause Damage?” on the YouTube.)

Happy driving!