Auto Academy: Do You Need to Change Your Transmission Fluid?

Auto Academy Banner

Honda Pilot drive buttons
Honda Pilot drive buttons | Micah Wright/Autos Cheat Sheet

It’s a proven fact that over time the fluids in your car will turn into a sludge-like, sticky brown substance that causes the problems it was meant to solve in the first place. There’s a reason why even today, getting one’s oil changed at regular intervals remains such a crucial part of automotive maintenance. Fortunately for us, modern automobiles don’t require transmission fluid changes nearly as frequently, but unlike oil, the debate over gearbox juice remains a rather prickly topic of contention.

As to exactly how often you should service your transmission really depends upon the manufacturer and vehicle in question, and even then you’re going to find a slew of different opinions online and from mechanics as to whether it even needs to be done at all. For most automakers, automatic transmission maintenance on many modern vehicles requires a complete flush every 100,000 miles or so, while some designs even going as far as eliminating the ability to check fluid levels entirely. While some mechanics may argue that this is entirely too infrequent, advancements in fluid detergent content and transmission cooling technologies have caused many to feel that as long as nothing is leaking from the bell housing, there is no need to even touch that drain plug.

Rear differential flush
Rear differential flush | Micah Wright/Autos Cheat Sheet

Manual transmissions are a completely different bag of bolts entirely, and depending upon mileage and driving style, may require more regular change intervals if shifting into third suddenly seems a bit iffy. Again, playing it safe and following the owner manual’s suggested maintenance schedule is always the best bet, especially when warranties expire and you are paying out of pocket.

Stop-and-go traffic, towing trailers, extreme temperatures, and hauling heavy loads in the back of a pickup will also accelerate the breakdown of gearbox fluid, so keep these factors in mind if you want to keep that drivetrain running smoothly. The harder your transmission has to work, the more muddy that red stuff will get, so if it comes out smelling burnt and looks almost black, then beware — that is a sure sign that your gearbox needs some attention.

Naturally, it’s also good to be on the lookout for abnormal shifting patterns, hesitation, and gear slippage, and don’t forget to make sure a fresh transmission filter gets on the vehicle, along with a new gasket, and a drain bolt crush washer. Speaking of drain bolts, it is never a bad idea to opt for a magnetically charged plug, as it will attract any shards of metal floating around inside, and only requires being wiped off when the flush is being conducted. Just be sure to get one that has a steel head on it; the last thing anyone wants is a cheap-ass aluminum bolt that rounds off and gets stuck after just a hint of torque.

Mazda6 shifter
Mazda6 shifter | Micah Wright/Autos Cheat Sheet

According to the Department of Motor Vehicles, “there are ways to drain and replace all the fluid in your transmission, but the simplest way for beginners is to do a partial replacement.” While the DMV does continue to urge amateurs to take their vehicle to a reputable auto mechanic instead of attempting a flush on their own, they do suggest that once you get some experience, you can “teach yourself to pump all the fluid out through the dipstick filler or one of the cooler lines.”

But before you go head first into this entire endeavor, hop online and see if there’s a YouTube video that specifically focuses on your vehicle and the maintenance you’re looking to be performed, because an informed DIY guy is an efficient one. If your vehicle comes with a dipstick give it a sniff and a peek; if it looks translucent and red, and smells like petroleum, you might not even need a full flush, leaving you with a partial change.

Castrol transmission fluid
Castrol transmission fluid | Micah Wright/Autos Cheat Sheet

But for most Americans, handing the keys over to an expert is the only option, and a lot of repair shops utilize quick flush equipment that makes the process even faster by forcing out old fluid and pumping in new liquid. While that may sound efficient and practical, several automakers strongly discourage this practice (Honda is especially outspoken). So do your research and find out if it’s OK to do a forced flush on your vehicle, because the last thing anyone wants is to be one of the thousands of Americans filing complaints against service stations.

Honda also requires that owners only utilize their branded transmission fluid and warn that using other brands could cause damage, which is a great way for a warranty to go up in smoke if your service records prove something else was added. There also is the issue with filters, and because some vehicles have them integrated inside the drain pan, having them cleaned or replaced when the fluid is changed often requires dropping the pan and re-gasketing everything.

Finally, there is the debate over whether a high mileage vehicle with 100,000 miles on the odometer or more should even have its transmission fluid replaced. An article by says that there are quite a few mixed opinions on this notion, “with some mechanics suggesting you should just leave [transmission fluid] well alone if you aren’t having shifting problems.” Having spoken with multiple mechanics on the matter over the years, we too have heard our fair share of stories about older transmissions failing after finally receiving their first flush, and the reasons why old fluid is the goo that holds it all together.

While some may have a hard time accepting the notion that fresh fluid causes transmissions to grenade on the interstate, the fact that thicker, older gearbox juice keeps leaky seals from seeping is well documented. Fortunately, you can forego this entire ordeal by simply following your vehicle’s recommended service intervals, and by replacing worn seals once detected.