Breaking Down the Science of Engine Oil
Taking Engine Oil Back to Basics
Forgetting about brands, it’s worth considering a few important things.
Firstly, a quick chemistry lesson! Viscosity measures a fluid’s resistance to flow, or in another sense it’s ‘thickness’. For instance, honey is more viscous than water. Engine oil needs to flow throughout your engine in both very cold and very hot temperatures, so viscosity is an important measure of an engine oil’s suitability.
Secondly, the engineers that designed the engine in your car generally had a specific oil in mind for use in your engine. You see, each engine has its intricacies – some engines have tighter tolerances or clearances than others – and the type of oil that you use can definitely affect engine performance, reliability, and even fuel economy.
The best way to figure out what engine oil is right for your car is to consult your owner’s manual to see what is recommended. Sometimes your vehicle manufacturer will recommend different oil types based on the outside ambient temperature – in Australia, we are lucky in that it doesn’t get too cold too often, but if you live in the more ‘extreme’ areas of climate variation (e.g. the Snowy Mountains) then it can be necessary to use a different oil in the winter.
Engine Oil Viscosity and Grades
Upon checking your owner's manual, it might say something along the lines of ‘use 5W-30 engine oil’. So what does that all mean?
Well, engine oil viscosity is clearly stated through this ‘5W-30’ term. In this instance, the first number (ie. 5) is the viscosity of the oil at a cold temperature (0 degrees F or -17 degrees C). The ‘W’ stands for winter. The second number is the viscosity of the oil at a hot temperature (212 degrees F or 100 degrees C).
So, a 5W oil will flow more easily than a 10W oil when cold. Similarly, 5W-40 oil will stay thicker at higher temperatures than a 5W-30.
Now you know what the numbers mean, what else is there to consider?
Mineral, Semi-Synthetic, or Full Synthetic? So Many Choices!
It isn’t just the viscosity of the oil that you need to determine, it is also the type of oil. There are two main types, mineral and fully synthetic. These terms refer to how the oils are produced; with mineral oils being produced in a relatively simple fashion as a product of crude oil, and synthetic oils being produced in a more advanced fashion with some additives thrown in. Semi-synthetic oils are usually a blend of the two.
Synthetic oils also have some added benefits in generally flowing better at colder temperatures and preventing oil breakdown at higher temperatures, helping to protect your engine from added wear. As a result of the more advanced manufacturing processes and aforementioned additives synthetic oils will nearly always cost more than mineral oils.
Without going too deep into the chemical engineering behind each type, the engine oil that’s best for your vehicle is nearly always the one recommended by its manufacturer. So, if your handbook says to use fully synthetic oil, then that’s the one you should use.
How Often Should I Change My Engine Oil?
Yep, you guessed it – check the owner’s handbook! Some vehicles require fresh oil as part of proper servicing every 5,000km, and some vehicles can be serviced every 25,000km if certain conditions are met. That said, changing your oil earlier can hardly hurt, but changing your oil after it has been in the engine too long can definitely be a sign of problems to come. Look for sludge under the oil cap, or in the oil pan after you have drained the oil. If sludge is present, consider changing your oil more frequently in the future.
One final word of caution – if (like me) you have a car that you don’t drive very often, it is best to change the oil based on time intervals rather than mileage travelled. Aim for every 6-12 months to be safe if your car is infrequently used. After all, regularly changing oil is a small price to pay for overall reliability and longer engine life.