This week for the technical article, I am going to describe oil viscosity grades. Several years ago, I decided to learn about them as I have always been curious as to the meaning of the grades on my oilcans! For example, my old Honda used 10W-30 – a multi-grade oil. The viscosity of a liquid is the resistance to deformation when under stress, such as when heat is applied as happens in an engine. So the lower the viscosity, the better the fluid will move – which is very important when a piston is rapidly moving in a cylinder.
Oils come in single or multi-grade and have been graded by the Society of Automotive Engineers (SAE). The grades are rated for low to high viscosity: 0 is the lowest and 60 is the highest and there are now many grades – the most recent revision (J300_201501) was released in January 2015 which is to be reviewed in a few years. Clearly with the advancement of technologies and materials, it is important for the SAE to review and update their own classification.
The oil used in the S2000 (and recommended by Honda) was a low viscosity oil (10 being a comparatively low number in the range). It’s important for an oil to be able to be moved quickly through the engine when it’s spinning up to 9,000 revs!
For single grades, the oil is tested at 100°C and the grade is determined based on its performance. For grades of 20, 30 and 40 it is then tested at 150°C for its performance. These grades are the most likely to be put into a typical road car. This brings me on to the “W” often seen in the grades. “W” is for winter and the lower grades are designed for the lower temperatures. The oil is rated after two tests that check the oil’s performance at different temperatures. The lowest temperature test that the oil passes is the one that it is rated at. So the oil I used has passed the lowest test at 10°C. This probably explained why the engine light came on in the middle of winter when I left it out overnight in the mountains. It hated the lower temperatures!
Oils can also be multigrade – the inclusion of the second number in the example of 10W-30 defines that it is a multi-grade. Multi-grades are the most common because the oil in a car’s engine typically has to run in a wide temperature range. The oils are tested at both the winter and normal temperatures and must pass both to be defined as a multi-grade. The idea is that the oil should have similar viscosity rating at the two different temperatures. If it doesn’t, additives are used to make them closer. Ideally, the oil should work well at both ends of the scale. The additives are known as Viscosity Index Improvers and help stabilise the oil across the range.
Whilst owning the Honda, I switched from the main brands of oil to the Nulon 10W-30 High Tech Fast Flowing oil. Not only was it cheaper, it also claimed to have 36% less friction than other comparable oils. It’s also Australian, so I was helping the local economy too. I figured that these were good reasons to switch – the lower friction interested me, as I thought that if the claim was true then the engine could have less wear.
It’s a very thin, honey coloured oil designed for high revving 4 cylinder motors just like in the Honda. It seems to be too thin, although in normal driving I didn’t notice any difference and I didn’t do any high rev tests to see if there was a change. By the time I sold the car the engine had done over 175,000 kms and was going great. I checked the oil regularly as part of my basic maintenance and topped it up – it used some if I just did town driving, however on the longer runs it didn’t use any at all.
It always important to review your car’s handbook to see what oil viscosity is recommended and to keep to that recommendation unless you are going to use the vehicle in a completely different way to the original intention – i.e. turning it into a full track car with no road use. The owners manual should also give guidance as to the oils to be used dependant on the typical weather conditions the car will experience.
Oil is a critical component to the longevity of an engine, so don’t ignore it!