Synthetic vs. Conventional Oil: 5 Myths Debunked
Independent AMSOIL Dealer — Bartlett, IL
Ask five different people at a Bartlett car show about synthetic oil and you'll probably get five different opinions, some of them decades old and never updated. Synthetic oil has been around since the 1970s, but the myths around it have stuck around just as long. Here are five of the most common ones, and what's actually true.
Myth 1: "Synthetic oil is just marketing, it's basically the same as conventional."
This one misses the actual manufacturing difference. Conventional oil is refined from crude, which means it comes with naturally inconsistent molecule sizes, impurities, and structural weak points. Synthetic oil is engineered at the molecular level, built for uniform molecule size and structure from the start. That consistency is what gives synthetic its real, measurable advantages: better resistance to heat breakdown, stronger film strength, and more stable performance across temperature extremes.
Myth 2: "Once you switch to synthetic, you can't switch back to conventional."
This myth probably outlived whatever caused it decades ago. Synthetic and conventional oils are compatible with each other and can be mixed or switched between without damaging your engine. That said, switching back to conventional after running synthetic means giving up the protection advantages you'd already gained, so it's less about "can you" and more about "why would you."
Myth 3: "Synthetic oil causes leaks in older engines."
This one has a real origin, which is part of why it's stuck around. Decades ago, some early synthetic formulations used seal-conditioning additives that behaved differently with aging gaskets, and a small number of older vehicles did see minor seep issues. Modern synthetic oils are formulated with seal-compatible additives that condition gaskets appropriately, for the vast majority of engines, synthetic oil doesn't cause leaks. If anything, an existing leak was often already present and simply became more visible with a thinner, more consistent fluid.
Myth 4: "You still have to change synthetic oil just as often as conventional."
This is one of the most costly myths for the wrong reason. Because synthetic oil resists oxidation, thermal breakdown, and viscosity loss better than conventional oil, it holds up over meaningfully longer intervals in many vehicles. Extended-interval synthetic products are specifically engineered for this. Your actual interval still depends on your vehicle, your driving conditions, and manufacturer recommendations, but assuming synthetic needs changing on the same schedule as conventional oil overlooks the entire reason synthetic exists. I recommend doing an oil analysis at your next oil change to see how much ‘life’ is still left in your oil. You can get an oil test kit from AMSOIL as well: Shop Used Oil Analysis Kits & Accessories - AMSOIL
Myth 5: "All synthetic oils are basically the same."
Not even close. "Synthetic" describes a manufacturing process, not a single formula, and the industry's labeling rules have made this murkier over the years, with some conventional-adjacent oils marketed as "synthetic blends" or under loose synthetic labeling. Base oil quality, additive packages, and specific engineering all vary significantly between brands and even between products from the same brand. This is exactly where doing some homework, or asking someone who works with these products daily, actually pays off. AMSOIL recently just improved its Signature Series as well.
The Bottom Line
Most of these myths trace back to either outdated information or a kernel of truth that got oversimplified over time. The short version: synthetic oil is a genuinely different, better-engineered product, it's compatible with switching from conventional, modern formulations don't cause leaks in normal circumstances, it supports longer intervals in the right applications, and not all synthetics are created equal.
If you've got a specific myth that you've heard and want a straight answer on, or you're not sure what's right for your vehicle, I'm happy to talk it through, no pressure. Learn more about becoming an AMSOIL Preferred Customer here: amsoil.com/p/preferred-customer-registration-preg/?zo=1129770