5 Synthetic Oil Myths That Mechanics Want You to Stop Believing

Synthetic Oil Myths Debunked: Engine Care Tips

When it comes to maintaining your engine, I’ve heard just about every synthetic oil myth in the book. As someone who has worked on engines for decades, I want to set the record straight. There’s a lot of misinformation out there, and these oil myths can actually lead people to make expensive, unnecessary, or even damaging decisions. Let me walk you through the facts and explain how to use synthetic oil properly.

Myth #1: You Must Flush Your Engine Before Switching to Synthetic Oil

I can’t tell you how many times I’ve heard this one. The idea behind it? That conventional oil is so bad that it creates sludge buildup in every engine. And, synthetic oil is so “strong” with detergents that it will instantly clean out your engine and dislodge sludge, supposedly causing huge chunks of carbon buildup and sludge to clog oil passages and destroy your engine.

Here’s the truth: If your engine is clean, a flush is unnecessary and does nothing helpful. If you’ve followed the carmaker’s recommendations for routine oil changes, your engine won’t have sludge buildup, even if it’s been using conventional oil its whole life. If you haven’t followed the oil change recommendations, then you’ve got far more serious engine wear issues than just carbon and sludge buildup.

Synthetic oil does have better detergents and dispersants than conventional oil. And it will slowly dissolve sludge buildup in your engine. If you know you have sludge buildup (pop off a valve cover and look at the condition of the rockers and cams), switching to synthetic oil and changing your oil more often is actually the safest way to clean out your engine. Synthetic oil will remove the deposits more gently than an engine flush.

#2 Switching to synthetic will cause my engine to leak — NOPE

This oil myth has its roots in the early days of synthetic oil, back when those products were built for race cars, not street cars. Back then, they didn’t include seal conditioners or detergents because professional race car engines are fully rebuilt after each race. When oil without conditioners was used in older vehicles with hardened/cracked seals, yes, synthetic oil did cause leaks. But that was over 25 years ago. All modern synthetic oils have seal conditioner additives.

However, since synthetic oils flow more easily when cold, if you already have cracked oil seals, switching to synthetic oil can cause those cracks to leak oil when the engine is cold.

Myth #3: You Can Automatically Extend Oil Change Intervals With Synthetic

I see this myth about synthetic oil repeated everywhere. Yes, synthetic oil holds up better under heat and stress. But your driving habits have more to do with oil life than mileage. Driving short trips, especially in cold weather, loads the oil with fuel, water, and soot. Because the trip is so short, the engine never gets hot enough to evaporate off the fuel and water, and the engine doesn’t run long enough to filter out the soot. That’s why carmakers list two different oil change schedules: one for normal driving and another for severe service. Severe service is also appropriate if you haul heavy loads, drive in mountainous conditions, drive in stop-and-go traffic, or drive less than 10 minutes in cold weather or less than 15 minutes in warm weather.

In the auto repair industry, we’re seeing large numbers of engine failures due to customers following the normal service schedule with synthetic oil when they should be using the severe service schedule. We’re seeing carbon buildup in the oil control rings, causing oil burning, timing chain stretch, excessive wear and clogging of variable valve solenoids and cam phasers. So, while it’s true that synthetic oil is more resistant to oxidation and thermal breakdown than conventional oils, it’s not a miracle fluid. Not following the right service schedule will cause engine damage, even if you use synthetic oil.

The only real way to know how to use synthetic oil for longer intervals is to do regular used-oil analysis. That means spending $25 or more per sample to check viscosity, additive breakdown, and contamination levels every few thousand miles. If you’re not willing to do that, stick with the interval recommended in your owner’s manual.

In my experience, trying to extend oil changes without testing is a false economy. Additives like antioxidants, detergents, and friction modifiers break down whether you drive hard or not. Even if the oil looks clean, it may not be protecting your engine like it should.

Myth #4: You Can’t Switch Back to Conventional Oil After Using Synthetic

Let me make this clear: this is just flat-out wrong. Your engine doesn’t have a memory. You can switch between synthetic and conventional oil as often as you like. There is absolutely no chemical incompatibility between the two.

If anyone tells you it’s harmful to go back and forth, they don’t understand how to use synthetic oil properly—or how engine oil works at all.

Myth #5: You Can Use a Thicker Viscosity With Synthetic for Better Protection

This oil myth sounds logical on the surface. Synthetic oil flows better at cold temps, so some people assume they can “safely” go to a higher viscosity—say, from 5W-30 to 10W-50—for better high-temp protection.

But here’s the problem: oil viscosity affects more than just protection—it also affects oil pressure and how variable valve timing operates. Using the wrong viscosity, even with synthetic, can trigger a Check Engine light, mess with timing, and increase wear.

Car makers spend millions researching the correct oil spec. Don’t try to outsmart them. If the manufacturer calls for 5W-30, then that’s what you should use, whether it’s synthetic or conventional.

Learning how to use synthetic oil the right way means following OEM guidelines—not “tweaking” the viscosity based on internet advice.

For more information on synthetic oil, see this post

© 2012 Rick Muscoplat

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