Synthetic Oil Or Conventional: Deciding Which Is Your Best Option

Discussing engine oil is a slippery subject. For starters, should you use conventional or synthetic oil? It used to be that the most troublesome choice to make when deciding on oil for your engine was the brand name on the label. But today much has changed. There are multiple technical areas and a ton of material to cover so let’s jump right into this viscous topic.

Technology around engine oil is constantly changing because late-model engines are evolving at a much quicker pace than in past decades. The discussion of conventional versus synthetic has been extensively covered. So instead, we’ll focus on oil application issues and how viscosity plays a part in that decision. To get some solid information, we reached out to the folks at Lucas Oil.

There are a bewildering number of choices for engine oil, but for the high-performance enthusiast, the best choice can typically be found at an auto parts store or a performance outlet that offers specialty lubricants for your application.

This is an especially important decision when it comes to older performance engines that are not adequately protected by current conventional oil. The current standard for engine oil as established by the American Petroleum Institute (API) is classified as SN. The API will tell you the latest SN designation is backward compatible with engines reaching back to the ’60s. That might be true for a 1965 six-cylinder engine with 8:1 compression, but certainly not even close for a big-cube V8 engine with mechanical flat-tappet lifters and aggressive valve springs. 

This is a small-block Chevy cam and lifter that failed because the owner used an API SN oil with a low ZDDP concentration. The lower 800 ppm concentration of ZDDP was not sufficient to prevent the destruction of the cam and lifters. The proper oil choice would be a 10w30 or 10w40 specialty lubricant offered by Lucas Oil, and many others that would have prevented this damage.

Conventional vs. Synthetic Oil — What’s The Difference?

Conventional oil comes from the distillation of crude oil pulled out of the ground. This conventional base oil product is made up of a large number of complex hydrocarbon chains that can look like a briar patch drawing when expressed as a chemical chain. To enhance the performance of conventional oil, refiners include additive packages that improve the oil’s viscosity, along with detergents, anti-wear components — like zinc and phosphorus (the full term is zinc dialkyldithiophosphate or ZDDP) — and multiple other things like ph neutralizers and anti-foaming agents.

Synthetic oil is essentially engineered to eliminate most of the extraneous branches of a conventional hydrocarbon chain. It is much “smoother.” By keeping the molecule simple, a synthetic lubricant will generally out-perform conventional oil concerning how it reacts to heat and pressure. There’s a lot of talk about synthetics engineered from natural gas. This is generally referred to with the acronym GTL or gas-to-liquid technology. There appear to be advantages to this process in case you want to look into that. “If the vehicle has an early style small or big block engine running a flat-tappet camshaft, I recommend using our conventional Hot Rod & Classic oil that contains the higher zinc level,” Says Tom Bogner, director of research and development at Lucas Oil. “The Hot Rod & Classic oil is available in 10W30, 10W40, and 20W50. This product can be run all season and fits the needs of most any small or big block. If you are running a hydraulic-roller cam, you may be just fine with an API-rated engine oil.”

Top 5 Oil Myths

Synthetic Oil Always Leaks

As with most urban myths, this one has a basis in truth. The first synthetics produced in the early ‘70s did leak, but this anomaly has been rectified with improved chemistry and the addition of seal swelling additives that minimize leaks. But keep in mind that any lubricant that reduces friction will be inherently more slippery. So there are no guarantees your engine will not leak with the addition of a synthetic. If your engine currently spills oil, a synthetic will not repair a mechanical problem.

You Can’t Switch Between Synthetic And Conventional

This is another inconsistency that common sense will override. With the popularity of blended semi-synthetics containing both conventional and synthetic base stocks, clearly, it’s not an issue if a user wants to convert from synthetic to conventional oil.

Use Diesel Oil To Break-In Flat-Tappet Cams

This used to be an accepted practice. But modern diesel oil formulations have radically lowered concentrations of zinc and phosphorus (ZDDP) that are now below the levels needed to ensure a proper cam break-in. Instead, choose a dedicated break-in oil with ZDDP levels of around 1,200 to 1,500 ppm to ensure the anti-wear additives will allow the cam and lifters to break in properly. An ideal break-in oil is one with very low detergents and the above ZDDP levels.

All Oils Use The Same Additive Package

A current API-specified product like an SN rating will have similar packages of detergents and anti-wear additives. Changes in viscosity do not necessarily change those packages. However, different brands will most definitely create their own additive package to the point where that a given brand will contend its additive package is superior. There are definite differences in additive packages between brands of oil, making the selection process a bit more complex.

Higher Oil Pressure Is Always Better

This falls under the “bigger is better” theory. However, in this case, it is not always true. Pressure is an indication of a restriction to flow. Pressure is necessary to ensure the oil reaches highly loaded bearings at all engine speeds. The standard of 10 psi per 1,000 rpm is acceptable but a properly built, normally aspirated performance engine will certainly live with less pressure. This also means a hot engine oil pressure reading of 15 psi at idle is more than sufficient especially since the engine is experiencing minimal load. This makes some enthusiasts uncomfortable but it is not necessarily an indication of a problem. 

Viscosity is a lubricant’s most important characteristic. This is defined as a liquid’s resistance to flow. So a low number like 10 or 20 grade refers to a relatively thin oil with a low resistance to flow. A higher number, like 40 or 50,  is a thicker oil that does not flow as easily.

Temperature plays a huge role in this evaluation. Think of oil like maple syrup. Right out of the refrigerator, it’s thick and flows very slowly. But heat it in a saucepan and it will pour more easily. The same thing happens to engine oil. As it heats up, its resistance to flow is reduced. The important thing here is that higher rated oil like a 40 grade will offer a thicker film of protection at a given temperature compared to a 20 grade. 

Viscosity is the ability of the oil to maintain a given lubrication barrier between loaded components like engine bearings and the crankshaft. Wider clearances will require thicker oil like a 20w50 instead of a 10w30. Tighter clearances can use lighter oil like a 10w30.

This is very important because the engine’s bearing clearances and application will determine the viscosity grade of oil you should choose. For example, a typical small-block V8 on pump gas will run with 0.0025-inch main and rod bearing clearance which would dictate 10w30 viscosity oil. Adding a supercharger, which increases the load on the bearings, would demand bumping the viscosity grade up to a 10w40 to provide more protection. Engines running on E85 or methanol also need a higher-viscosity grade since these fuels run rich air-fuel ratios and will tend to thin the oil through dilution. 

“When selecting a viscosity, I recommend you start by asking your engine builder what he suggests,” says Bogner. “The trend these days is to use lighter oils. Back in the day, just about everyone used 20W50 conventional. A 20W50, whether semi-synthetic or full synthetic, might be the right choice for a Trophy Truck racing 500 or 1,000 miles, but that oil would probably be robbing horsepower and not providing the best lubrication for your street car or classic.”

What Is This ZDDP Stuff?

With all the changes to oil these days, most car guys are aware that today’s API SN category oil has radically reduced ZDDP levels to meet newer emissions standards. When zinc and phosphate are combusted, they tend to coat the precious metals in catalytic converters which will lower their efficiency. However, a non-emission-controlled performance engine often needs a higher level of ZDDP to be adequately protected, which demands a specialty oil to provide these higher levels.  

These specialty oils offer higher concentrations of ZDDP for use with flat tappet cams to ensure that the camshaft or lifter does not fail. However, a new performance engine owner may not be aware of the fact that his flat-tappet lifters require higher levels of ZDDP, and the use of SN category off-the-shelf oil will eventually fail the cam and/or lifters as a result. This means those enthusiasts should avoid oil that carries the current API donut listing SN. To help in this search, some companies list their ZDDP numbers as concentrations in parts per million (ppm). A comfortable level of ZDDP for a flat-tappet cammed performance engine would be roughly 1,200 to 1,500 ppm. Current API SN oil has a ZDDP limit of 800 to 850 ppm. 

Viscosity is important to keep the crankshaft away from the bearings. This is what happens when the oil film breaks down due to high temperatures or excessive load. Luckily, this engine just tagged the tri-metal bearings but didn’t kill the crank or rods, which saved the engine.

Many of these specialty oils are a conventional makeup, although there are some synthetic blends worth considering. A synthetic oil blend is a mixture of conventional and synthetic base oil with the synthetic helping improve the performance of the conventional base oil. But the term synthetic covers a wide range of different types of base oil and mixture percentages, so you really have to do your homework to know the differences. It’s beyond the scope of this story to dive too deeply into this, but within the realm of synthetics, the most common is Group IV base oils, which are polyalfaolefins (PAO). For synthetic blend oil, there is no standard for the percentage of mixture of synthetic and base oils. This means a blend could have as little as 10-percent added synthetic base stock. 

What’s In A Number?

Viscosity is defined as the rating of oil’s ability to flow at a given temperature. In the 1960s, before the days of multi-viscosity oil, the procedure was to use lighter viscosity oil during winter months to allow the oil to flow easily during startup. This might be an oil with a 20-grade viscosity rating. Conversely, summer months required thicker viscosity oil like a 30 or 40. Today, viscosity index improver additives allow multi-viscosity oil such as a 5w30, 10w30, or 10w40 that thicken when exposed to higher temperatures. 


synthetic oil

Lucas’ Hot Rod & Classic Car 10W-40 is manufactured with paraffinic base oils and is fortified with a unique additive package containing high levels of zinc, molybdenum, and phosphorus, which provides a tougher, thicker additive film for maximum protection even under the most severe conditions.

Current new car manufacturers are lowering the viscosity target by choosing progressively thinner oil for production engines. European engines are now specifying 0w16 oil in an attempt to improve fuel economy. While some enthusiasts might be tempted to run lighter viscosity oil in a performance engine to improve power, this can lead to bearing problems if the thin oil allows the high load to push past the thin barrier of oil between the crank and the bearings. 

We performed a simple viscosity test on the dyno by progressively testing lighter oils, ranging from 10w30 to 5w20 in a 6.0-liter LS engine that was designed to run 5w30 oil. We saw a minor improvement of a little over 3 horsepower average from 4,000 to 6,000 rpm. However, we did not evaluate whether there was an increase in wear due to the lower viscosity. The prevailing engine builder position is that bearing clearances establish the viscosity of oil to be used. If we decided to push this engine harder by making more power with a supercharger, we would need to increase the viscosity to ensure the bearings and crank were protected.

Another way to achieve greater protection without increasing the viscosity is to step up to synthetic oil. Companies like Lucas are using what is called an mPAO base oil synthetic and have proven this high-quality base oil offers increased protection with a thicker film base, even at elevated engine oil temperatures of 300 degrees.

In discussions with the people who make oil filters, a filter engineer once commented, “The best oil filter is a really good air filter.” Engines ingest tons of air and with cleaner air, the oil will remain cleaner, longer. This includes filtering the air for the PCV system.

As a further example, we recently watched an LS3 with a roots blower dyno test on the internet churn out 850 hp on pump gas. The standard recommended oil for an LS3 is a 5w30. Assuming normal bearing clearances, this supercharged application would demand moving up to a minimum of a 10w40 oil and seriously consider a full synthetic or at least a synthetic blend to provide high-load protection for the bearings. With street engines now pushing 1,000 hp on gasoline, you would not want to make the mistake of pouring a basic SN API grade oil into an engine like that.

There’s a ton more material on oil than we can present in this short story but hopefully, we’ve introduced some new ideas on how to manage the lubrication side of your next engine project. Beyond just protection and lubrication, with the right selection of viscosity and oil grade, you can even make a little more horsepower while protecting that engine so it lives a long prosperous life.

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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