The American Petroleum Institute, API, is a U.S.A trade association for the oil and natural gas industry, and have set up the industry standard for the energy conservation of the motor oil.

There are two categories: petrol and diesel. Category S is for “Spark Ignition”, for petrol and category C is for “Compression Ignition’, for Diesel. For both, the letter S or C is followed by another letter. For example, SM or CF. The higher the letter, the more recent the technology of the oil. The second letter in the code is critical to read before buying. This is because it indicates the vehicle model year the oil was formulated to service.

The API donut on the right tells you if the oil meets the current SL service rating. It also provides the SAE (Society of Automotive Engineers) viscosity number and tells you if the oil has passed the Energy Conserving test.

 Viscosity

Viscosity refers to a fluid's resistance to flow. In motor oil, it's rated at zero degrees Fahrenheit ( -17,78°C) (represented by the number preceding the "W" [for winter]) and at 212 degrees (100°C) (represented by the second number in the viscosity designation). Motor oil thins as it heats and thickens as it cools. So, with the right additives to help it resist thinning too much, an oil can be rated for one viscosity when cold, another when hot.

The more resistant it is to thinning, the higher the second number (10W-40 versus 10W-30, for example), and that's good. Within reason, thicker oil generally seals better and maintains a better film of lubrication between moving parts.

At the low-temperature end, oil must be resistant to thickening so that it flows more easily to all the moving parts in your engine. Also, if the oil is too thick, the engine requires more energy to turn the crankshaft, which is partly submerged in a bath of oil. Excessive thickness can make it harder to start the engine, which reduces fuel economy. A 5W oil is typically what is recommended for winter use. However, synthetic oils can be formulated to flow even more easily when cold, so they are able to pass tests that meet the 0W rating.

Once the engine is running, the oil heats up. The second number in the viscosity rating—the "40" in 10W-40, for example—tells you that the oil will stay thicker at high temperatures than one with a lower second number—the "30" in 10W-30, for example. What is important is that you use the oil viscosity your car's owner's manual recommends.

Conventional Oils or Synthetic Oils?

If you look on Start My Car, you will see we carry a full range of oils, and that they all kinds of specific purposes: high-tech engines, new cars, higher-mileage vehicles, heavy-duty/off-road SUVs. In addition, you will see a wide selection of viscosities.

If you read your owner's manual, you will know what the car manufacturer recommends for a brand-new vehicle. However, as cars get older or perhaps based on the temperatures where you live, you may want to put in a more suitable grade.

Conventional Oil

These oils are made for high-tech engines. If these oils pass stringent special tests (indicated by their labelling), it means they have superior, longer-lasting performance in all the critical areas, from viscosity index to protection against engine deposits. They flow better at low temperatures and maintain peak lubrication at high temperatures. While excellent oil, synthetics are about three times as expensive as conventional oil and not always necessary for most engines.

Use the owner's manual as a guide. If it does not call for synthetic oil, using it will only be an additional expense that may not add anything to the engine's performance or life.

 Part Synthetic Oil:

This is essentially premium conventional oil hit with a dose of synthetic. They are formulated to offer better protection during heavier engine loads and the associated higher engine temperatures. These oils are popular with pick-up and SUV drivers because they do offer better protection, but usually cost only a fraction more than premium conventional oils.

Higher-Mileage Oil:

Today's vehicles last longer, and if you like the idea of paying off the car and running the mileage well into six figures, you have another oil choice, those formulated for higher-mileage vehicles.

More than 65 percent of vehicles on the roads in South Africa have more than 75,000 miles (120 000 kilometres) on the odometer. Playing to this growing market, oil refiners and labs developed high-mileage oils. Seal conditioners are added to the oil (the oil can be synthetic or conventional) to expand and increase the flexibility of internal engine seals. The conditioners are very precise and can benefit some engines while not affecting others.

The seal conditioners that flow into the pores of the seals to restore their shape and increase their flexibility. In most cases, rubber seals are designed to swell just enough to stop leaks. But the oil refiners pick their "reswelling" ingredients carefully. Higher mileage oils also have somewhat higher viscosities. They also may have more viscosity-index improvers in them. The result? They seal piston-to-cylinder clearances better and will not squeeze out as readily from the larger engine bearing clearances. They also may have a higher dose of anti-wear additives to try to slow the wear process.

Summary

Vehicle engines are usually designed with a specific type, viscosity, and amount of oil in mind. The oil will match the engine's close tolerances, move through the galleries in the correct way and take away heat at a measured pace. It will also match the type of driving the car or truck is primarily used for; for instance, a daily driver will take different oil than a work truck. Use the guidelines in this article to ensure that you choose the correct oil for your vehicle!