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Full synthetic oil is often sold as the gold standard for engine protection. It flows better in cold weather, resists breakdown at high temperatures, and lasts longer than conventional oil. But it’s not always the right choice - and using it in the wrong situation can cost you money, hurt performance, or even damage your engine.
Your Engine Was Designed for Conventional Oil
Many older cars, especially those built before the mid-2000s, were engineered to run on conventional motor oil. The seals, gaskets, and internal clearances in these engines were designed with the thicker, slower-flowing nature of conventional oil in mind. When you switch to full synthetic, which is thinner and more slippery, it can cause oil leaks. That’s not because the synthetic oil is bad - it’s because the seals have hardened over time and no longer swell to fit the tighter tolerances synthetic oil creates.
Classic car owners and mechanics in Adelaide regularly see this. A 1987 Toyota Corolla with 180,000 kilometers on the clock might start dripping oil after a synthetic oil change. The fix? Go back to a high-mileage conventional blend or a synthetic blend that’s less aggressive. Synthetic oil doesn’t hurt these engines - it just exposes weaknesses that were already there.
You’re Breaking in a New Engine
New engines need a proper break-in period. During the first 1,000 to 2,000 kilometers, the piston rings need to seat properly against the cylinder walls. This process relies on a bit of controlled friction. Full synthetic oil is designed to reduce friction as much as possible - which sounds great, but it actually prevents the rings from seating correctly.
Manufacturers like Ford, Chevrolet, and Subaru still recommend conventional oil for the break-in period on new vehicles. Even if your owner’s manual says “synthetic oil recommended,” check the fine print. Many say “synthetic blend or conventional” during the first oil change. Skipping this step can lead to higher oil consumption, reduced power, and lower fuel efficiency down the road.
Your Vehicle Has a Flat Tappet Camshaft
Older performance engines - think classic muscle cars, older V8s, or modified engines with aftermarket camshafts - often use flat tappet lifters. These parts rely on zinc and phosphorus additives (ZDDP) in the oil to prevent premature wear. Full synthetic oils, especially newer formulations, have reduced ZDDP levels to protect catalytic converters and meet modern emissions standards.
Without enough ZDDP, the cam lobes and lifters can wear out in as little as 5,000 kilometers. You’ll hear a ticking noise, then lose power. Replacing a camshaft in a V8 can cost over $2,000. That’s why many engine builders still recommend a high-ZDDP conventional oil or a dedicated break-in oil for these engines. Some brands even sell synthetic oils specifically formulated for flat tappet cams - but standard full synthetic? Not safe.
You’re Driving a High-Mileage Vehicle with Known Leaks
If your car has over 150,000 kilometers and you’re already topping off oil every 1,000 kilometers, switching to full synthetic is asking for trouble. The oil’s superior cleaning properties can wash away sludge that’s acting as a temporary seal in worn valve guides, oil pan gaskets, or crankshaft seals. Suddenly, you’re losing a liter of oil every 500 kilometers.
High-mileage oils are specially blended with seal conditioners and thicker viscosity modifiers to slow leaks. They’re not perfect, but they’re designed for this exact problem. Full synthetic? It’s like using a pressure washer to clean a cracked windshield - it removes the grime, but the crack stays, and now it’s worse.
You’re Running a Budget-Conscious Daily Driver
Full synthetic oil costs 2 to 3 times more than conventional oil. If you drive a 2008 Honda Civic with 220,000 kilometers, and you change your oil every 10,000 kilometers, you’re spending $120 a year on synthetic versus $40 on conventional. That’s $80 a year you could put toward new tires, brakes, or even a future down payment.
Modern conventional oils are far better than they were 20 years ago. They meet API SN, SP, and even some newer standards. For a low-stress engine that doesn’t tow, doesn’t race, and doesn’t sit in traffic for hours every day, conventional oil still does the job. You don’t need synthetic to get 300,000 kilometers out of a well-maintained engine.
Your Vehicle’s Manual Specifically Says Not To
Some manufacturers still advise against full synthetic in certain models - especially older or niche vehicles. Jeep’s 2005-2007 Liberty with the 3.7L V6, for example, warns against using full synthetic in its service manual. Why? Because the engine’s oil pump and internal passages were calibrated for the viscosity and flow characteristics of conventional oil. Using synthetic can cause low oil pressure at idle, triggering warning lights or even engine shutdown.
Always check your owner’s manual. If it says “conventional oil only” or “do not use full synthetic,” there’s a reason. Manufacturers don’t say that lightly. They’ve tested it. They know what happens when you ignore it.
You’re Using the Wrong Viscosity
Synthetic oil doesn’t fix bad viscosity choices. If your engine needs 10W-40 and you use 5W-30 synthetic because it’s “better,” you’re creating problems. Thinner oil might flow easier, but it won’t protect under load. In hot climates like Adelaide, where summer temps regularly hit 40°C, using a too-thin oil can lead to metal-to-metal contact in the bearings or camshaft.
Always match the viscosity grade your manufacturer recommends. Synthetic oil just makes that grade perform better - it doesn’t let you cheat the rules.
When Synthetic Oil Is Worth It
There are plenty of cases where full synthetic is the smart choice: turbocharged engines, vehicles that tow heavy loads, cars driven in extreme cold or heat, or engines that sit idle for weeks. If your car is under warranty, and the manufacturer recommends synthetic, use it. If you drive 30,000 kilometers a year and want to stretch oil changes to 15,000 kilometers, synthetic gives you that flexibility.
But don’t assume it’s always better. Oil is not a one-size-fits-all product. It’s a precision tool - and like any tool, you need to pick the right one for the job.
What to Do Instead
If you’re unsure, start with a high-mileage synthetic blend. It gives you some of the benefits of synthetic without the risks. Or stick with conventional oil and change it more often - every 5,000 to 7,000 kilometers. That’s still cheaper than synthetic, and it keeps your engine clean and protected.
Keep your maintenance log. Note when you change oil, what type you used, and if you notice any leaks or unusual noises. Over time, you’ll learn what works for your specific vehicle - and you won’t waste money on oil that doesn’t need to be there.
Can I mix synthetic oil with conventional oil?
Yes, you can mix synthetic and conventional oil in an emergency. Both are made from base oils and additives that are compatible. But it’s not ideal. Mixing them dilutes the performance benefits of synthetic and doesn’t improve the protection of conventional oil. It’s better to stick with one type and change it properly.
Is synthetic oil bad for older engines?
Not inherently. But in high-mileage engines with worn seals, synthetic oil’s cleaning power can remove sludge that’s acting as a seal, leading to leaks. It’s not the oil’s fault - it’s exposing existing wear. For older engines, use a high-mileage oil blend instead.
Does synthetic oil cause engine sludge?
No, synthetic oil resists sludge better than conventional oil. It’s more stable at high temperatures and doesn’t break down as easily. If you see sludge after switching to synthetic, it was already there - the oil just cleaned it up.
How often should I change synthetic oil?
Most manufacturers recommend changing synthetic oil every 10,000 to 15,000 kilometers, or once a year, whichever comes first. But if you drive in dusty conditions, tow trailers, or frequently make short trips, change it every 7,500 kilometers. Synthetic lasts longer, but it doesn’t last forever.
Can I use synthetic oil in a motorcycle?
Some synthetic oils are safe for motorcycles, but not all. Motorcycle engines share oil with the clutch and transmission, so you need oil rated for wet clutches (look for JASO MA or MA2). Automotive synthetic oils can cause clutch slippage. Always use oil labeled for motorcycles.
Final Thoughts
Full synthetic oil is powerful - but power without context is dangerous. Just because it’s expensive or marketed as premium doesn’t mean it’s right for your car. Know your engine’s history, its age, its condition, and what it was built for. The best oil isn’t the one with the fanciest label - it’s the one that matches your engine’s needs.
