How to baffle people by being helpful.
Good, clean oil is vital to an engine, but there are so many types and brands these days that it’s hard to tell which is the right one for you. Getting it wrong can be costly, so here’s a brief history and explanation of the various numbers and codes, together with three simple rules to follow to make sure the oil you pour into your engine is the right one.
Oil has to do some very important work inside an engine. The most obvious function is to lubricate the moving parts. An engine relies on lots of bits of metal moving up and down or rotating at very high speeds, and for it all to work properly they have to fit very closely together. But if they touch, friction will very quickly heat the surfaces and they will melt. So, oil is pumped around all the moving parts to provide a continuous thin film between them to stop them touching.
The second purpose oil serves is to carry heat away from the moving parts, heat from friction and the heat from the burning fuel. And finally, it has to mop up a lot of nasty stuff produced as a by-product of the burning fuel and store it in suspension until the next oil change – you might view an oil change as the automotive equivalent of a trip to the loo!
In order to do all this the oil has to be thick enough for the thin lubricating film to stay intact at very high pressures to withstand all the buffeting which goes on – bear in mind that a cylinder “firing”, is actually a mixture of fuel and air exploding and you’ll get the idea. And, of course it has to stay thick as it heats up. At the same time it has to be thin enough to circulate freely, especially when the engine’s cold.
So how thick is thick?
The thickness (or viscosity) of oil is indicated by the “SAE number”, and is usually called the “Grade” of the oil. The higher the number the thicker the oil. (Don’t even think of asking what the number relates to, just trust me that 90 is a lot thicker than 20).
Single grade oils were the norm years ago, but that often meant changing oil for thinner stuff to last the winter and now multigrades are what you get for car engines. The SAE number is the one with two figures separated by a “W”, as in “10W40” “5W30” and so on. “W” stands for winter and for “10W40” it means that the oil acts like a 10 grade when it’s cold and like a 40 grade when it’s hot. A clever cocktail of chemicals thicken as the temperature goes up, countering the thinning of the oil.
The second number is the important one, since you don’t want the oil to be too thin at high temperature, so if the handbook says 15W40, a 10W40 will be fine, but a 15W30 will make the engine sound like a bag of nails when it’s hot. If in doubt, 10W40 will be fine for most modern cars (i.e. from about 1990 onwards) in the UK. It is certainly fine for an emergency top up on any car if you suddenly find you’re low on oil mid journey.
As well as the thickening chemicals, the oil will contain additives to improve pressure resistance, neutralise the toxic combustion products and help prevent sludging. In general, the more expensive the oil, the better the additive pack. In a bid to simplify things, the oil industry came up with a quality rating – the right rating guaranteed that the oil would contain all the additives needed to make it do everything necessary for the full period between oil changes.
Great idea. Unfortunately, there are two bodies defining quality standards for oil, the API in America and the ACEA in Europe and they both use different ratings. As if that wasn’t confusing enough, there are separate ratings for petrol and diesel. API ratings begin with S for petrol and C for diesel engines, whereas ACEA ratings use A for petrol and B for diesel – and they tell you entirely different things as well.
Confused? Not surprising. Some oils carry the API symbol (see below) to make life a little simpler, but there’s no requirement to do so.
So, how do you know which to use? Fairly simple actually. Just look for the latest quality specification letters on the container. At the moment they are “A3”, “A5” or “SM” if you have a petrol engine, “B3”, “B5” or “CI4” if it’s diesel. If the oil has petrol and diesel specifications, it’s fine for both engine types.
Non-branded oils often just show the numbers, so you may well see something like “10W30 A3 B3 SM CI4” – and you’re supposed to work out the rest for yourself. The later the oil, the higher the number and later spec oils are always compatible with earlier ones, but NOT the other way around. So, you can use A3 oil in an engine where A2 was specified, but you must not use A2 if the handbook says A3. The big name oils always conform the latest spec and will therefore be fine for any engine – again, from about 1990 onwards.
Then there’s the synthetic/mineral divide.
The nineties saw huge changes in engine design – fuel injection, variable valve timing, higher compression pressures, temperatures and speeds and a host of emission reduction measures put even greater demands on the oil, to the point where the stuff produced by Mother Nature could barely cope.
So, enter synthetic oils. Synthetic oil is produced by men in white coats in laboratories, who design it specifically to meet the demands of modern engines. As a result, they can make very thin oils (5W30 or even 0W30 are normal) which can be pumped round the engine at high speed at about the consistency of tap water, but still do the job of protecting, cooling and cleaning. This is great for high performance engines, but the down side is the price – perhaps double that of mineral oil, so there’s a halfway house which is semi-synthetic, a blend of synthetic and mineral, which does give good protection at a reasonable price. Grade numbers and quality standards are indicated in exactly the same way, so you can get a synthetic, a semi synthetic and a mineral oil, all specified as 10W40 C5.
The semi synthetic is more than adequate for all but the most exotic engines and extreme driving conditions. However, high performance engines increasingly specify fully synthetic, so, as ever, check the handbook and ALWAYS use the oil type, quality and grade specified by the manufacturer.
This has become particularly important in the last few years as some manufacturers developed their own specifications – and failing to use oil that is specifically labelled as conforming to them could invalidate the warranty. It seems to have started VW and their “PD” or “Pumpe Duse” engines – fitted in VW, Audi, Seat and Skoda cars and other VW derived types such as the Ford Galaxy. The spec was 505.01, as opposed to their 500.00 for “normal” diesels. To start with, you had to buy this at VW dealers, but these days the big name brands will conform to the latest VW specification.
Nevertheless, it is vital to check the car’s handbook and then double check the oil container you’re thinking of buying. VW is now up to spec 507.00 and above. The latest spec is always suitable for previous cars, PROVIDED the grade is correct, so you don’t need to search around for old dusty cans of 505.01 for your 2001 Golf TDi.
Other (mainly German) manufacturers jumped on the “our cars need special oil” bandwagon, so also look for BMW specs starting with LL, such as LL.04, Merc’s 229.3, 229.4 and so on and GM/Vauxhall with their GM-LL-B-025 format specs.
The important thing is to check the owners’ handbook, or ask at the dealer if the car doesn’t have one, then check the oil can before you buy.
Finally, if the oil doesn’t have SAE or ACEA numbers, it’s best avoided, even if it’s endorsed by a sporting hero or a celebrity from Big Brother!
The rules for choosing the right oil:
- Check the handbook for the right oil type. If you don’t have one, enter your car details into one of the internet oil sellers and they’ll come up with the grade and quality of oil you need. www.commaoil.com/ is a good one because it gives all the specifications. www.opieoils.co.uk or www.millersoils.com are another couple to try.
- Make sure the grade is correct, particularly the second number.
- Look for the latest quality specification (A3, A5 or SM for petrol engines, B3, B5 or C14 for diesels).
- Semi synthetic oil works well in most cars, and is often specified for cars made in the past 10 years. Fully synthetic oil is pricey and not really necessary for normal cars, unless it’s specified by the manufacturer.
- Check that the manufacturers own specification is shown on the oil container if your car requires it (mainly German makes).
- If the grade and quality specs match your car, the make isn’t too important. To an extent, you get what you pay for, but a cheaper lesser known brand with the right markings will do the job perfectly well. Top brands may give higher performance in racing engines or in gruelling conditions, but for most of us, that level of protection is much more than we need. Regular oil changes are far more important than an expensive brand name.
Words: Tim Shallcross