By: Gov Auctions | 22 September 2017

A Guide to Choosing the Right Motor Oil for your Car

gov-auctions - Choosing the Right Motor Oil

With so many types of motor oil on the market, choosing the right one for your vehicle is really a process of elimination. It involves:

  • Checking your owner’s manual for the recommended viscosity range.
  • Taking into consideration where you live and the extremes of temperature your car will be operating in.
  • Considering the type of vehicle you drive including its age and mileage.
  • Selecting an oil that meets all of these requirements.

This guide looks at how oil works and what it contains, the importance of viscosity in the equation, the types of oils that are available, and the questions you should ask yourself when choosing the right oil for your vehicle.

How engine oil works

Oil performs a number of vital roles in an engine. Its main job is to create a thin film that separates the components of the engine to prevent metal-on-metal contact as they rotate, slide and pound against one another.

Oil also creates a sealant around the pistons in the cylinders, helps to cool moving metal parts which become heated due to friction, neutralises acids, prevents metal corrosion and carries dust, dirt and combustion by-products to the oil filter, where they are captured and removed when the filter is changed.

In short, without oil to lubricate, cool, clean and seal, the engine would run for a matter of seconds and then seize up entirely, so it is a crucially important ingredient in the modern combustion engine.

What oil contains

Oil is primarily organic oil (anywhere from 70 to 95%) and the rest is made up of man-made additives. These are used to improve and maintain oil performance and a number of different types of additives are added to oil including:

  • Viscosity index improvers – help prevent the oil from thinning with increasing temperature.
  • Detergents - keep the engine surfaces clean by inhibiting the formation of deposits, rust and corrosion.
  • Dispersants – help to disperse solid particles so they can’t combine to form sludge, varnish and acids.
  • Anti-wear agents – protect metal surfaces when the lubricating film breaks down.
  • Friction modifiers – reduce engine friction and improve fuel economy.
  • Pour-point depressants – prevent the wax particles in oil from congealing and reducing flow at low temperatures.
  • Antioxidants – prevent oxidation and thickening of oil at high temperatures.
  • Foam inhibitors – used to collapse foam bubbles created by the crankshaft.
  • Corrosion inhibitors – protect metal parts from acids and moisture.


One of the main considerations when choosing a motor oil is its viscosity index. Viscosity is a fluid's ‘resistance to flow’ and in the case of oil, it becomes less viscous (thinner) as temperature increases and more viscous (thicker) as it cools.

Additives are used to help reduce the extremes of this thinning and thickening process and create an oil that can be used at both ends of the temperature scale.

When you look at the markings on a can of oil, you will see its viscosity designation represented as ‘XW-XX’. The first number before the W rates the oil’s flow at 0 degrees Fahrenheit, the W stands for Winter and the last two numbers rate the oil’s flow at high temperatures.

So for instance, if the viscosity index on one can is 5W-30 and 10W-30 on another, this means that the first can (5W-30) thickens less in the cold than the second (10W-30). And if one can says 5W-30 and another says 5W-40, the 5W-40 is more resistant to thinning at high temperature than the 5W-30.

So in order to choose an oil that’s right for your vehicle, you need to check the owner’s manual for the recommended viscosity range, factor in where you live and what extremes of temperature your vehicle will be exposed to, and then select an oil that will operate effectively within these parameters.

Types of oil

The wide range of oils available mean that other things need to be considered as well as viscosity. Oil types on the market include:

  • Conventional oil – the cheapest oil containing few additives. Because of this, it is better suited to vehicles which have frequent oil changes.
  • Premium conventional oil - the standard oil used in most new cars and available in the common viscosities.
  • Full synthetic oil - made for high-tech engines, offering superior, longer-lasting performance in all the critical areas. It can cost 3 times as much as conventional oil, so may not be worth the cost, unless recommended by the vehicle’s manufacturer.
  • Synthetic blend oil - premium conventional oil formulated to offer better protection during heavier engine loads and higher engine temperatures. Popular oil with pickup and SUV drivers.
  • High mileage oil – made for older vehicles, this higher viscosity oil contains seal conditioners to make aging internal engine seals more flexible and additives to improve film strength.

How to choose

A good way to select the right oil for your vehicle is to ask yourself these questions:

  • What kind of oil have you been using in the past and is it really necessary to choose a new type or better to stay with what you know?
  • What kind of oil does the manufacturer recommend in your owner’s manual?
  • Where do you live and is the area subject to extremes of temperature? (important for viscosity)
  • What is the mileage on your vehicle and would it benefit from a high mileage oil?

If you take these factors into consideration, you should be able to make an educated guess as to which oil is best for your vehicle. Remember to always pay attention to any recommendations in your owner’s manual and if in any doubt at all, be sure to consult your local mechanic before making a choice, particularly if your vehicle is relatively new, as changing to a different type of oil could void your car warranty.