For as long as motorists have been maintaining their automobiles, there have been numerous theories and ongoing controversy about which engine oil to use. In the early days, oil choice was primarily brand dependent; often based on what your father and his father used in the family’s grocery getter. Today, brand loyalty aside, our options range from conventional to synthetic oils, and semi-synthetic to extended-life oils. Since the engine is one of your vehicle’s most expensive components, it is important to select the oil that will help it last as long as possible.

With so many choices, selecting the “best engine oil” has become more confusing than ever. In this article I’d like to give you some basic oil facts, so that the next time you’re staring at the “wall of oil” in your local auto parts store, making the choice that’s best for your vehicle will be a little easier.

Conventional vs Synthetic Oil – Which is Best?

Is conventional oil just as good as a full-synthetic oil when it comes to helping your engine last longer? The answer to that question is … it depends. Maintenance habits, environmental conscience and the manufacturer’s recommendations have a lot to do with determining which oil is right for you. Some people swear by conventional oil and think full-synthetic oil is a waste of money. Others won’t put anything but full-synthetic oil in their engines, even if it’s not mandated by the manufacturer. Let’s explore the benefits and drawbacks of both synthetic and conventional oil so you can make an informed choice.

Purity – Full-synthetic oil is 100 percent man made and free of any impurities. Synthetic oil contains no crude oil and is created from pure, uniform chemicals in a controlled environment. Being impurity-free means better protection of vital internal engine parts in extreme temperatures and loads and for much longer periods of time than conventional or semi-synthetic oils. Full-synthetic oil is usually recommended for high-performance or turbocharged engines and for vehicles towing or hauling heavy loads.

Conventional oil on the other hand, comes from the ground and is littered with contaminants such as rock fragments, minerals, naphthenic acid, sulfur and salt water which needs to be removed from the base oil before the additives can be introduced at the refinery. Although they do a good job, the refineries can’t remove all the contaminants.

Wax is something that is also not removed. Varying amounts of it remain in all conventional oil. Wax causes the oil to thicken in cold temperatures and can lead to a lack of lubrication on cold start-ups. If you’re not diligent with your oil changes, wax can lead to sludge, which blocks oil passages and leads to premature wear and engine damage.

Environmental Concerns – Fewer oil changes provide major benefits to the environment. Used motor oil is a major source of toxic waste in water. One gallon of used motor oil can degrade one million gallons of water. The old oil change standard of every 3,000 miles is sadly out of date and no longer applies to most vehicles … even when using conventional oil. Many federal, state and local environmental agencies advise adhering to your vehicle’s manufacturer-recommended oil change intervals. Easily check your vehicle’s oil change intervals in the service schedule of your owner’s manual or use a reliable online resource. The California Recycling website is a great example. Here’s the link: https://www2.calrecycle.ca.gov/UsedOil/ChangeInterval/

In all cases, the manufacturer’s oil change intervals when using full-synthetic oil is about twice that of conventional oils. That significantly reduces the amount of waste oil and its negative impact on the environment.

Wear and Tear – As I mentioned, with full-synthetic oil, you can go about twice as long between oil changes than with conventional oil. As a senior master technician who has been changing engine oil for over 45 years, I’d say this is a very important advantage. Changing oil less often reduces the chance of stripping the oil drain plug threads or leaving the plug loose, oil filter leaks and making a mess on the garage floor or anywhere else for that matter. Also consider the time you’d save not having to dispose of used oil and the added expense of doing twice as many oil changes. The bottom line? Although full-synthetic oil costs more than conventional oil, you gain the benefits of superior lubrication over a longer time period. And, fewer oil changes mean less wear and tear on oil change parts and fewer chances for potential mistakes.

Test Results – Over the years, there have been numerous studies done comparing full-synthetic oil to conventional oil. One was done recently by the American Automobile Association (AAA). They employed a variety of industry-standard tests on newer vehicles with turbo-charged engines and on vehicles that frequently drove in stop-and-go traffic, towed heavy loads or operated in extreme hot or cold conditions. These conditions are listed as “severe” in your vehicle owner’s manual and apply to just about any vehicle driven on the road today. AAA’s findings aligned with most other studies - Synthetic oil outperforms conventional oil, is more resistant to deterioration, flows better in cold conditions, protects better in hot weather and is worth the extra up-front price.

The auto makers have done their testing too and have been using full-synthetic oil in their turbo, high-performance and naturally aspirated engines on the assembly line. They have concluded that full-synthetic oil protects their engines better in a wide variety of conditions and reduces the overall cost of ownership. Also, because of full-synthetic oil’s superior lubricating properties, a lower viscosity can be used, which helps manufacturers tighten up the tolerances on internal engine parts and meet federal fuel mileage and emissions requirements.

Can You Mix Full-Synthetic and Regular Oil?

In a word … Yes. You can safely mix synthetic and conventional oil. Some oil manufactures do exactly that and call it “semi-synthetic” or “synthetic blend” oil. However, while you can mix full-synthetic and conventional oils, you are diluting the performance benefits of full-synthetic oil. So, if you’re traveling and find your engine oil is a quart low, adding a quart of conventional oil to your full-synthetic oil will safely get you home and to the next oil change.

Synthetic Oil Will Make Your Engine Leak

This is not true today, but this urban legend is based on some fact. The early synthetic oils created in the 1970s was formulated using chemicals that were harder on seals. And, because the seal materials were often not compatible with those chemicals, the resulting leaks gave synthetic oil a black eye. Modern synthetic oils are much more advanced and use chemicals that, not only are compatible with seals, but actually conditions them to maximize their life expectancy. Switching to a synthetic oil in an older, high-mileage vehicle that has used conventional oil for years may result in oil leaks. But it’s not the motor oil’s fault. In this case, the seals may be already dry, cracked or loaded with sludge. The advanced detergents in full-synthetic oil will quickly clean away the sludge (which is keeping the seal from leaking) and instigate a leak.