Leave the square seal on the old oil filter to prevent leaks

Oil changes are the kind of task you do without thinking too much—until you forget the tiny piece that keeps everything from dripping out. That piece is the square seal, the rubber gasket that sits on the oil filter. If you’re working on cars long enough, you’ll learn that the devil is in the details, and this one is a classic example.

Why that little seal matters

Think of the oil system as a closed loop: oil is pumped through the engine to lubricate parts, then returned to the sump. The oil filter sits in the loop to catch the gunk, but for it to do its job, the filter has to seal against the engine block. The square seal—usually shaped like a tiny O-ring or a round rubber gasket—creates that barrier. If there’s even a hairline gap, oil will seep out, the pressure will drop, and you’ll be staring at an oily mess under the hood. In a worst-case scenario, a poor seal can lead to low oil pressure, poor lubrication, and accelerated engine wear. Not something you want to explain to a customer, right?

What stays on the old filter—and what doesn’t

Here’s where things get a little confusing if you’re not paying attention. When you remove the old filter, you want to take the old seal with it—then clean the mounting surface. The old filter should come off with its seal intact. If a portion of that square seal sticks to the engine block, you’ll end up with two seals trying to do the same job. That double-seal can create a leak or a misfit that defeats the purpose of the new filter.

Now, the other parts you hear about—magnetic tape, pressure relief valves, thread sealers—don’t have the same role in the context of replacing the filter. Magnetic tape isn’t a standard feature on oil filters. Some filters do have a bypass or pressure-relief valve built into the unit, but you don’t leave that valve behind on the engine block; it’s part of the filter assembly. Thread sealers aren’t used on oil filters either—the filter threads onto the engine with a gasket seal, not with paste or sealant. The square seal is the star of the show here, because it’s the direct line between the filter and the engine.

A quick moment to reflect on the consequences

If you neglect the old seal, a couple of bad things can happen. First, you’ll get a leak. Even a slow drip can create a slick under the car and a reminder to check the oil level more often than you’d like. Second, you might end up with air pockets in the filter, which can compromise filtration and oil flow. And if the seal is partially left behind, you could get a leak right when you’re most hopeful—after you’ve topped up the oil and cleaned up. All of that is avoidable with a simple check.

A practical, no-nonsense change-out routine

If you’ve done this more than a few times, you may have a practical routine that saves you time and headaches. Here’s a clean, straightforward approach you can follow, with a few honest tips thrown in.

• Gather the right gear: new oil filter (with a fresh O-ring), the correct filter for the vehicle, the right oil, a catch pan, rags, a wrench that fits the filter, and a small amount of oil for lubrication.

• Prepare the workspace: warm oil is friends with you when you drain it, but not so hot that you burn yourself. Position the car on a level surface, engage the parking brake, and slide under with caution.

• Drain the old oil: position the catch pan under the drain plug, remove it, and let the oil flow. Don’t rush this—patience now means less cleanup later.

• Remove the old filter: twist the filter off by hand or with a filter wrench. Keep a rag handy because oil will splash a little.

• Inspect the mounting surface and the old seal: look for any sign that the old square seal remained on the engine block. If you find it, remove it with a plastic scraper or a rag. You want a clean, smooth surface for the new seal to seat against.

• Check the new filter: verify the new unit has a fresh O-ring. Lightly oil the O-ring with new engine oil so it seats smoothly and doesn’t bind as you tighten.

• Install the new filter: hand-tighten first, then give it another quarter to half turn (or follow the filter’s torque spec in the manual). Do not over-tighten; you’ll risk damaging the seal or threads.

• Refill and check: pour in the new oil, run the engine for a minute, then check around the filter for leaks. If you see a weep, shut off, tighten a touch more, and recheck. Clean up any drips so you don’t chase oil leaks from a dirty floor.

• Final check: after a few minutes with the engine off, recheck the oil level and top up if needed. A quick road test can confirm everything is quiet and leak-free.

Tips from the trenches

• Don’t assume a filter with the same thread size will fit every engine. The seal size and shape can vary. If you’re swapping a filter brand, verify that the gasket is compatible with your engine block.

• If the old seal is stubborn or has stuck to the engine, don’t pry at it with a metal screwdriver. That’s a fast way to nick the mounting surface. A plastic scraper or a fingernail can do the job without risk.

• If you’re dealing with a pressure-relief or bypass valve, remember it’s part of the filter assembly. You don’t service it separately during a simple oil change; you replace the entire filter.

• Hand-tighten is a friend. The moment you feel more resistance than a gentle snug, stop. Overtightening is a frequent cause of leaks and damaged seals.

• Keep a small mirror nearby for those hard-to-see spots under the engine. A quick glance can reveal a hint of oil where it shouldn’t be.

Relatable digressions that still stay on point

Oil filters aren’t glamorous, but they’re the quiet guardians of the engine’s heartbeat. It’s kind of like changing a guitar string—you don’t think about it when it’s in tune, but if you forget it, you’ll hear a squeak that distracts from everything else. The seal is the same way: invisible most of the time, but essential for keeping the orchestra in harmony. And since you’ve got a mix of folks who pick cars for practical reasons and others who love the engineering puzzle, there’s something satisfying about getting that tiny O-ring to cooperate with the rest of the system.

Real-world flavor: brands you’ll hear about

• Many DIYers reach for MOPAR, Bosch, Fram, AC Delco, or Motorcraft filters. Each brand has its own quirks in gasket thickness and surface finish. The common thread is this: the O-ring is the star, and a fresh one with the correct size is your best bet for a leak-free seal.

• If you see a filter that’s unusually tight or emerges with a stubborn feel, that’s your cue to pause. Check for cross-threads or a damaged gasket. It’s not a badge of honor to force it on; it’s a quick way to invite a leak.

Common myths—and the little truths that cut through them

• Myth: You can reuse the old gasket if it looks fine. Truth: A used gasket rarely seals as well as a new one. If there’s any doubt, replace it with the new filter’s O-ring and a fresh bead of oil.

• Myth: A tiny oil drop somewhere after you start the engine isn’t a big deal. Truth: It is if it’s the seal doing the talking. A few drops may mean a leak that worsens as hours pass.

• Myth: The pressure-relief valve is the main seal. Truth: It’s a critical component, but it’s part of the filter unit itself, not something you separate during a routine change.

Why this matters for the broader engine health

And here’s the bigger picture. When you keep the oil system tight and clean, you’re protecting the engine’s internal components—bearings, camshaft lobes, and valve trains—all of which rely on consistent lubrication. The square seal may be a small component, but it sits at a critical juncture. It’s a reminder that maintenance is a collection of tiny decisions, each with consequences that add up over time. Don’t underestimate the power of a good seal.

Closing thought: respect the tiny things

The oil filter’s O-ring is a perfect example of how the most unassuming parts can carry the weight of the whole system. It’s a reminder to treat small details with respect, to double-check, and to keep a tidy workspace. If you approach an oil change with that mindset, you’ll not only keep engines happier—you’ll also feel more confident the next time you reach for the filter wrench.

So next time you change the oil, give that square seal its due. Give the old seal the boot, make sure the mounting surface is clean, and install the new filter with a fresh gasket and a touch of oil. It’s a small move with big payoff, and it keeps the ride smooth, the engine quiet, and the day a little less messy. And honestly, that’s a win worth tightening for.