Perhaps one of the most commonly asked questions in automotive maintenance, as well as one of the most frequently misunderstood, is “What engine oil is best for my car?” Set foot in any auto parts store, and you’re likely to be met with immediate information overload – rows of bottles, a confusing array of viscosity grades, and various types of oil that all claim to be the right choice – the selection can be intimidating. Thankfully, the difference between the three main types of oil – conventional oil, synthetic blend, and full synthetic oil – extends well beyond just the price, and while choosing the wrong oil for your engine can have real consequences down the line, we’re here to clue you up and help you learn how to discern between three different types of automotive oil.
Once the terminology makes sense, making the right decision becomes pretty straightforward. In fact, most modern engines have already made the decision for you, the answer to which can easily be found in your vehicle’s owner’s manual. But if you’re driving an older Honda Accord that has been handed down, then a little bit of research, like what we’re about to provide, might be worth it.
Oil type recommendations, change intervals, and performance comparisons in this article are drawn from manufacturer technical guidance published by Castrol, Mobil, and Pennzoil, alongside OEM service documentation and CarBuzz’s own maintenance editorial. General interval recommendations are intended as a guide only – always refer to your vehicle’s owner’s manual for exacting figures and guidance.
What Is Regular Oil And Who Still Needs It?
Also referred to as mineral oil or conventional oil, regular oil was the first type of automotive oil ever developed. Based on refined crude oil, this variant is generally found in the engines of older vehicles. Thanks to its lower heat-resistance and oxidation, this is where the 3,000-mile oil change rule of thumb originated. While the refining process removes many impurities from crude oil, it doesn’t substantially alter the molecular structure of the actual hydrocarbons, thus resulting in a less uniform base oil that is ultimately more prone to degrade under sustained heat and loads.
Conventional oil is still appropriate for older and more mechanically simple engines that operate under normal driving conditions such as light use in moderate temperatures or relatively low mileage accumulation per year. For the vast majority of cars on the road today, however, manufacturers no longer recommend conventional oil as the minimum standard, rather opting for some form of synthetic oil instead. In many cases, using regular oil where synthetic oil has been specified will likely void the engine warranty.
What Does Full Synthetic Oil Mean, And What Does It Do?
Full synthetic oil is engineered at a molecular level from synthetic base stocks. These are either Group IV polyalphaolefins (PAOs) or Group V esters as opposed to simply being refined from crude oil. The resultant base fluid has a far more uniform molecular structure than anything that might be achievable through conventional refining – it is this uniformity that underpins every aspect in which full synthetic motor oil excels over its alternatives. These aspects are the clear advantages if you’re wondering to yourself, “What does full synthetic oil do?”
“Synthetic oils are more resistant to varnish build-up and heat degradation, and have improved lubricity properties which lead to less drag and wear on engine internals.”
Is full synthetic oil better? Yes and no; it depends on the application. Practically speaking, full synthetic oil provides superior engine protection across a wider operating range with better cold-start performance, the oil flowing more quickly at low temperatures, exhibiting better resistance to thermal breakdown at high temperatures and cleaner engine internals thanks to more effective detergent additive packages, as well as longer drain intervals. In addition, the higher thermal stability of full synthetic oil can extend oil change intervals to 10,000 miles on some modern cars, a stark contrast to the 3,000-mile baseline mentioned.
What Is Synthetic Blend Oil And Where Does It Fit In?
Synthetic blend oils are a combination of conventional mineral oil with synthetic oil, often featuring an additive package, too. The ratio of synthetic to conventional oil varies by brand. However, conventional oil will usually make up the majority of the oil blend. The resultant combination sits somewhere between full synthetic and conventional oil in terms of performance, offering better cold-weather flow and improved oxidation resistance compared to normal mineral oil, but still not quite as resistant to thermal degradation and with a shorter drain interval than a full synthetic oil.
The practical use case for a synthetic blend vs full synthetic is someone that needs to make use of some of the benefits provided by synthetic oil and more protection than what conventional oil might provide, but one whose vehicle might not mandate the use of a full synthetic oil. This could be light trucks and SUVs that operate under moderate loads, as well as some turbocharged compact cars where full synthetic oil might not necessarily be required by the OEM. For the more budget-conscious owners of older, naturally aspirated engines in moderate operating conditions, a good-quality synthetic blend represents a notable step up from using conventional oil, yet without the cost of switching to a full synthetic alternative.
Full Synthetic vs Synthetic Blend vs Regular Oil
Castrol notes on its US technical guidance page, “a full synthetic motor oil formulated with a robust additive system will always be better than synthetic blends. That said, a properly formatted semisynthetic can offer performance benefits over a conventional motor oil.” The table below is a breakdown of the practical differences relevant to motorists across the five most relevant criteria for everyday ownership.
|
Practical Differences Between Oil Grades |
|||
|---|---|---|---|
|
Regular Oil |
Synthetic Blend |
Full Synthetic |
|
|
Engine protection |
Basic |
Moderate |
Superior |
|
Change Interval |
~3,000 miles |
~5,000–7,500 miles |
~7,500–15,000 miles |
|
Cold-start performance |
Poor |
Moderate |
Excellent |
|
High-temperature resistance |
Low |
Moderate |
High |
|
Approx. cost per quart |
$5–$8 |
$8–$12 |
$10–$20 |
Is Full Synthetic Oil Worth It For Your Vehicle?
If you’re wondering, “Should I use full synthetic oil?” the easy answer for most drivers is yes – and indeed, for a growing number of vehicles, it’s no longer optional. Users will find that most modern turbocharged engines, which now account for the majority of new car sales in the US, require full synthetic motor oil as a minimum standard. With the advent of Variable Valve Timing systems in most engines that require sufficient oil pressure and flow to operate, as well as tighter engine tolerances, synthetic oils have given rise to the era of longer service intervals. What this means, however, is that making use of conventional oil where synthetic oil has been specified risks sludge formation, premature wear and, in the case of VVT-equipped engines, inadequate oil pressure being sent to the variable timing solenoids, thus causing timing faults and the potential for expensive repairs.
As for whether or not full synthetic oil is worth the premium for vehicles that don’t require it, a simple cost calculation can answer that question. While full synthetic oil may cost more per quart, it also lasts significantly longer between oil changes, in addition to its superior cleaning and wear-protection properties which undoubtedly extend engine life. For high-mileage engines, there is one caveat that applies: the often impressive cleaning properties of full synthetic can dislodge deposits in the engine that have built up around gaskets and worn seals, potentially causing leaks in engines that previously showed no symptoms when running on conventional oil. In cases like this, a synthetic blend engineered specifically for high-mileage engines is often the best choice.
Should You Ever Switch Between Oil Types?
There is an often quoted myth that switching from full synthetic oil back to conventional oil may cause engine damage, but this has no basis in fact. Conventional oils and synthetic oils are compatible and can be mixed without chemical volatility or interference, though for continued use, it is recommended to use an oil type that is matched to the OEM’s specification as opposed to alternating between oil types. Perhaps the only non-negotiable rule when changing the type of oil used is that the API specification and viscosity grade must meet those as listed in the vehicle’s owner’s manual. While upgrading to a full synthetic oil where a blend was previously used is always acceptable, downgrading to conventional oil in an engine that specifies synthetic oil is ill-advised.
The Right Oil For The Right Engine
Deciding on what oil one should put in one’s vehicle isn’t quite as complicated as the shelves of the auto parts store might have you believe. For the majority of vehicles built in the last fifteen years, full synthetic oil is either the clear best choice or is mandated by the OEM, offering longer intervals, cleaner engine internals and better protection all-round, easily justifying the price premium when keeping and maintaining a vehicle over the long-term. Conventional oil does still have a place, but in older, simpler engines, synthetic blends make sense to bridge the gap between older and more modern vehicles. And should you be in doubt, the owner’s manual will always have the right answer.
Sources: Castrol US, Mobil US, Pennzoil.