“If your $20-per-quart synthetic looks dirty on the dipstick, you’d better change it now, ’cause all mechanics say dirty oil will ruin the engine. It’s science!”

“Speaking of, were you on the forums today? There’s an oil thread—yeah, another one—about how Europe gets way better synthetic oils than we do. Yeah, German cars get a Group 69 ultra-hyper synthetic made from plants harvested at the summit of the Zugspitze, and we Americans have to get by with sludge-filled dino oil. You believe that nonsense?”

“Whatever. I’m gonna change mine at 500 miles to be safe, then throw in three bottles of additives, ‘cause the zinc’ll turn my LS7 into an L88.”

How many times have you heard some variation of these oil “tips”? There’s the timeless “change it every (insert comically low mileage interval here)” advice. And the always popular “for the love of God and Team America, never let a drop of non-synthetic touch your oil pan” gem. Our guess is, you’ve heard ’em lots of times. We have too, and believed more than a few of them. Why is that?

Because like many other high-tech products today, motor oils are extremely difficult to understand—and unlike a smartphone, you can’t just take oil apart. You can ogle the port work on a new set of heads and read their dyno results, or fondle the strong shaft of a billet shifter (as this is a lubrication article, we’ll stop right there). But trying to understand oil quality is like trying to figure out Walter White: you have no clue about what’s going on in there, and can only hope that it’s not going to blow something up.

Complicating matters further, when the GF-4 oil standard came out awhile back, it drastically reduced the zinc and phosphorus used in oil for anti-wear protection. While newer, roller-valvetrain Vettes from later C4s to C7s aren’t affected, it’s a serious concern for owners of earlier, flat-tappet-valvetrain models.

Yes, we Corvette fans ask lots of questions about oil. So VETTE asked a few oil experts to help us explain it: The geniuses at Mobil 1, Driven Racing Oil, and Brad Penn assisted us right-brain types in getting our heads around this intimidating topic so you could as well.

We won’t pull punches: This tech goes deep. But once you have a general understanding of motor oil, you’ll realize just how much cutting-edge technology lives in that quart bottle. Let’s get started.

Once you have a general understanding of motor oil, you’ll realize just how much cutting-edge technology lives in that quart bottle.

Part I
The Refining Process

Crude oil contains thousands of compounds. Thanks to refineries, companies are able to use different refining processes that turn crude oil into everyday items such as lubricating oils, fuels, plastics, and waxes.

How Crude Oil is Refined

Three major refinery processes—separation, conversion, and purification—turn crude oil into finished products.

Separation is a distillation process that uses a series of separation (or distillation) towers and heat to physically separate crude oil into its naturally occurring components.

In the separation towers, a furnace heats and vaporizes the crude oil. The vapor/liquid mix is fed into the bottom of the tower, where temps can reach 750 degrees (F). Components that are still liquid at those high temps become the tower’s bottom product. Components in vapor form rise up through a series of distillation stages. As the temps decrease, the components condense and end up in their predetermined spots. So the heavy stuff like “bottoms” (which becomes asphalt base) stays on the bottom, mid-weight stuff like gas oil and diesel distillate (which become gasoline and diesel) goes in the middle, and lightweight stuff like “light ends” (which becomes propane) goes to the top.

But gas oil and diesel distillate doesn’t just “become” the gasoline and diesel that we pump into our vehicles. That’s where conversion comes in.

Conversion processes take low-demand products like heavy oil and rearrange the molecules into high-demand products like gasoline. This is possible because all of the products in the refinery are based on hydrocarbons with carbon and hydrogen chains.

Special units called fluidized catalytic crackers (FCCs), cokers, and hydrocrackers are used to “cut” the heavy oil’s longer carbon chains into shorter chains, which converts heavier hydrocarbons into lighter ones like gasoline. Additionally, there are catalytic reformer and alkylation conversion processes that can put these chains together, and even change the form of the chains.

Other conversion processes include delayed cokers, which convert the heaviest vacuum tower components into other products, and catalytic reforming, which increases the octane number of gasoline blends.

The purification stage physically cleans unwanted substances out of the product. This is done by hydrotreating, a process that puts the product in contact with hydrogen, then uses heat and high pressure in the presence of a catalyst. The result is hydrogen sulfide—and desulfurized product. While sulphur is the main target, aromatic hydrocarbons and paraffin wax are removed too.

How Lube Distillates Become Base Oil

Now let’s narrow down the refining process to lubricants:

After crude oil passes through the separation towers, it’s sent to cracking units to break up the hydrocarbons’ long carbon chains. This creates a group of carbon compounds that have between 25 and 45 carbon atoms, called lube distillates.

Next comes processing, where solvent extraction, dewaxing, or hydrotreating is used:

  • Solvent extraction is a conventional process that removes aromatics.
  • Dewaxing is either a conventional solvent process or a catalytic chemical process that removes long-chain, high-melting paraffins.
  • Hydrotreating is a hydrogen treatment that removes carbon-to-carbon double bonds in aromatics and unsaturated paraffinics.

The processing methods can vary, depending on the required quality of the finished base oil. For instance, a solvent extraction method is used for Group I oils, and a more advanced hydrocracking method is used for Group II and III oils.

Base oils that are more highly processed have a higher purity, a higher group number, higher performance, and, usually, a higher cost. We’ll have more on oil base-stock group numbers in a bit.