By Pat Lannone
Every motor oil — whether conventional, synthetic blend, or fully synthetic — contains a package of additives designed to improve performance and prevent engine wear. Understanding what’s in your oil and which type your engine needs is one of the most important parts of basic car care.
Why Oil Changes Matter
Changing your oil and oil filter on schedule is essential maintenance, and choosing the right oil for your engine is just as critical. Always consult your owner’s manual before buying — using the wrong oil can damage your engine and may even void the warranty on a new or rebuilt one.
The easiest way to confirm you’re picking a quality product is to look for the American Petroleum Institute (API) Certified Starburst on the bottle. The accompanying API “donut” tells you how the oil performs and lists its viscosity rating from the Society of Automotive Engineers (SAE). Many of today’s performance oils also help conserve fuel.
Understanding Viscosity
Viscosity — the thickness of the oil — is the single most important factor when choosing a motor oil.
- Too thick: the oil won’t flow freely through the engine’s moving parts, leaving them vulnerable to friction, contaminants, and overheating.
- Too thin: the oil can’t coat engine components properly and offers little protection.
Ambient temperature also plays a major role. Oil thickens in cold weather and thins as temperatures rise, which is why cold-weather starts can be sluggish and burn extra fuel before the engine reaches optimal operating temperature. Additives are blended into modern oils to keep them stable across this range.
The numbers on the API donut tell you exactly how the oil behaves in both cold and warm conditions. In an SAE rating like 5W-30, the suffixes (0, 5, 10, 15, and 25) paired with the letter “W” designate the oil’s winter grade. Your owner’s manual will specify the right grade for your engine — 5W-30 is the most common today.
Types of Motor Oil
Conventional motor oils work well in newer vehicles used for short commutes and light-duty driving, where the engine doesn’t run especially hot. They contain chemical additives and come in a range of viscosities. If you use conventional oil, plan to change the oil and filter every 4 months or 4,000 miles.
Synthetic motor oils are put through stringent quality-control testing and perform reliably at both hot and cold extremes. They resist evaporation, have a better viscosity index (meaning their viscosity changes less with temperature), and offer stronger protection against contaminants and overheating. Synthetics contain a higher percentage of additives than conventional oils and are considered top-tier products — and some manufacturers specifically require them.
Synthetic blend motor oils combine synthetic and organic oils. They’re built to resist oxidation and overheating, making them ideal for heavy-duty vehicles that haul loads, tow trailers, or drive on rough terrain. They also help with fuel economy.
High-mileage oils are formulated for older, well-worn vehicles. They include additives that hold viscosity better and protect against wear, plus seal conditioners that help prevent leaks around aging bearing seals and restore compression in the combustion chamber.
What’s Actually in Motor Oil: The Additives
A typical motor oil is roughly 75–85% base oil and 15–25% Performance Additive Package. The base oil itself helps manage particulates and oxidation, while the additives enhance performance, protect the engine, and stabilize viscosity. Manufacturers tailor these formulations to meet the specifications set by carmakers and engine builders.
Viscosity-index improvers. Polymer additives that keep the oil stable across a wide temperature range, so its viscosity doesn’t swing dramatically from a cold start to a hot highway run.
Dispersants. These trap and suspend solid contaminants in the oil so they can’t damage the engine, slowing the formation of sludge, acids, and varnish.
Detergents. Working on hot zones like the piston ring area and piston under-crown, detergents prevent deposits, rust, and corrosion. They’re consumed gradually as the oil ages.
Anti-wear agents. These coat high-stress engine parts — cylinder walls, piston rings, lifters, and cams — with a protective film that reduces metal-on-metal friction. They double as antioxidants and deplete over time.
ZDDP (zinc dialkyl-dithiophosphate). A zinc-and-phosphorus compound that was once a standard anti-wear additive, ZDDP has been scaled back because it can damage catalytic converters. However, older vehicles with flat-tappet camshafts or modified engines running higher-than-stock spring pressures still need oils that contain it to properly lubricate cam lobes, lifter bodies, and lifter bores. For reference, older SL-rated oils contained 1,000 ppm or more, while SM-rated oils are capped at around 800 ppm.
Friction modifiers. Compounds like graphite or molybdenum reduce friction under high temperatures and heavy loads, which also improves fuel economy. They lose effectiveness over the oil’s service life.
Antioxidants. High engine temperatures cause oil to react with oxygen, leading to premature aging, thickening, and sludge. Antioxidants slow this oxidation, keep the engine cleaner, and extend the oil’s useful life.
Anti-foam additives. Foamy, aerated oil can’t coat or cool engine parts properly, which can cause serious damage under high temperatures. Anti-foam additives stop air from getting compressed into the oil — even when the crankcase is accidentally overfilled. This is especially important in engines with variable camshaft timing, where the oil also functions as hydraulic fluid.
Rust and corrosion inhibitors. Moisture and acids inside the engine can corrode internal parts. These additives lay down a protective film to shield those surfaces.
Pour-point depressants. In cold weather, wax particles in oil can harden and resist flow. Pour-point depressants keep the oil flowing freely at low temperatures, so the engine doesn’t have to work harder to pump it.
Dexos. A specification developed by General Motors engineers for use in GM engines. Dexos oils deliver better fuel economy, reduced emissions, and longer emissions-system life. They feature improved anti-foam properties, resist sludge buildup, help protect the catalytic converter, and typically require less frequent changes than standard oils.
API Engine Oil Service Categories (Gasoline Engines)
| API Grade | Zinc (ppm) | Phosphorus (ppm) | Status | Service Description |
|---|---|---|---|---|
| SN | 400–750 | 400–700 | Introduced October 2010 | For 2011 and older vehicles. Improved high-temperature piston deposit protection, stricter sludge control, and better seal compatibility. API SN with Resource Conserving matches ILSAC GF-5 by adding improved fuel economy, turbocharger protection, emission-system compatibility, and protection for engines running ethanol blends up to E85. |
| SM | 400–870 | 400–800 | Introduced November 30, 2004 | Improved oxidation resistance, deposit protection, wear protection, and low-temperature performance over the oil’s life. Some SM oils also meet ILSAC specs or qualify as Energy Conserving. Can be used where SJ and SL are recommended. |
| SL | 1,000–1,400 | 1,000–1,400 | 2001 Gasoline Engine Service | Adopted for 2001 and earlier passenger cars, SUVs, vans, and light trucks under manufacturer-recommended maintenance. Tested per ACC Product Approval Code of Practice. Can be used where SJ and earlier are recommended. |
| SJ | 1,000–1,400 | 1,000–1,400 | 1997 Gasoline Engine Service | Adopted in 1996 and first mandated in 1997. For present and earlier passenger cars, vans, and light trucks. Can be used where SH and earlier are recommended. |
| SH | 1,000–1,400 | 1,200–1,400 | Obsolete | Model year 1996 and older engines |
| SG | 1,000–1,400 | 1,200–1,400 | Obsolete | Model year 1993 and older engines |
| SF | 1,000–1,400 | 1,200–1,400 | Obsolete | Model year 1988 and older engines |
| SE | 1,000–1,200 | 1,200–1,400 | Obsolete | Model year 1979 and older engines |
| SD | 1,000–1,200 | 1,200–1,400 | Obsolete | Model year 1971 and older engines |
| SC | 1,000–1,200 | 1,200–1,400 | Obsolete | Model year 1967 and older engines |
| SB | 1,000–1,100 | 1,200–1,400 | Obsolete | Model year 1963 and older engines |
| SA | 0 | 0 | Obsolete | Model year 1930 and older engines |
Valve Rocker Lifter Cam Diagram
| 1 – Valve 2 – Rocker Arm 3 – Valve adjustment screw 4 – Valve adjustment locking nut 5 – Push rod 6 – Hydraulic and non-hydraulic lifter 7 – Flat tappet camshaft and cam lobe |