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Five things you didn't know about your car's oil

Carlton Boyce / 26 December 2015 ( 28 August 2018 )

What type of oil should you use in your car? Is cheap engine oil as good as the expensive brands? Five things you probably didn't know about car engine oil.

Oil cap on a car
Oil is a tricky business with so many types, grades and viscosities

Checking and topping up your car’s engine oil was easy back in those halcyon days when we first learned to drive: we just bought a can of engine oil from your local garage and added enough to bring the level up to the mark on the dipstick.

There was no confusion as to what sort of oil to put in either, because life was binary back then: The Beatles or the Rolling Stones, BBC or ITV, Ford or Vauxhall, and Castrol GTX or Duckhams Hypergrade. (We also got very good at doing it because cars used a lot of oil back then and if you didn’t check it every week you ran the very real risk running dry and seizing your engine…)

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Things are more complicated now; there are different oils for diesel and petrol engines, turbocharged and non-turbocharged plus a variety of grades and viscosities. 

Of course, it’s not all bad news because modern cars tend not to burn as much oil as they did back in the day, but that just makes it all the harder to check the oil on the rare occasions when you need to.

Let's try and make some sense of it all, shall we?

What those numbers mean

Castrol GTX (yes, I was a Castrol man) was an SAE 20W50, which simply meant that the Society of Automotive Engineers (SAE) classified it as a ‘20’ thickness in the winter when it was cold (yes, ‘W’ stands for winter!) and a ‘50’ thickness in the summer when it was hot.

This is important because an oil needs to be thin enough to allow the engine to start when the oil is heavy and treacle-like in the depths of winter, while remaining thick enough to lubricate the engine properly when it is very hot after running for hours in a traffic jam at the height of summer.

So now you know what the numbers mean you can better understand what oil your car needs. 

Modern engines run much closer tolerances than older engines so need thinner oil. So whereas a 20W50 was common back in the 1960s and 1970s, you are now more likely to need a 0W30, or a 5W40. As ever, check your car’s handbook for details.

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What about the letters?

The eagle-eyed among you will have noticed that oil containers have letters as well as numbers. The most common are those assigned by the API, or American Petroleum Institute. These two-digit letters will start with an ‘S’ (for spark, ie, a petrol engine) or a ‘C’ (for compression, ie, a diesel engine), followed by another letter that relates to the year it was introduced. 

So an SG, for example, means that it was introduced in the early 1990s and is suitable for engines built before that date. The most recent classification is SM, which is suitable for all modern engines.

The European Automobile Manufacturers Association (ACEA) provides the equivalent European specification. The ACEA marking starts with an ‘A’ for petrol engines, ‘B’ for diesel, ‘C’ for cars with catalytic converters, and ‘E’ for heavy-duty diesel engines. The latest number is each category is A5/B5, C4 and E9.

As with viscosity, your car’s handbook will tell you which oil meets your car’s requirements and you shouldn’t deviate from that recommendation.

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Will cheap engine oil do the job?

The two key pieces of information are the SAE viscosity rating and the API/ACEA code. As long as those two are being met then yes, cheaper oil should do the job as well as more expensive oil.

However, there is one caveat to that: counterfeiting of engine oil is a problem, so only buy from a reputable retailer or garage because the £5 bottle of oil that’s being sold at your local car boot sale might not be all it claims to be…

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Synthetic, semi-synthetic or mineral oil

The Castrol GTX we used to buy a few decades ago was a mineral oil, which simply means that it comprised base mineral oil plus additives. It did a fine job in the cars it was designed for and still would in a simple classic car like a Morris Minor, Mini, or MGB.

Synthetic oil is created in a laboratory and has been honed to provide the best possible protection under specific circumstances. It is much more expensive as a result, but will give better protection and lasts for much longer. 

Some cars run for 20,000 miles or more between services, so need the very best oil to cope with such an extended service interval. (Do you remember changing your engine oil every 3,000 miles?)

Semi-synthetic oil is a combination of the two and is almost a meaningless designation dreamt up by marketing teams. They will usually contain less than 30% synthetic oil, with the rest being plain old mineral.

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What should I use in my classic car?

If you are lucky enough to own a classic car, then modern engine oils might be a bit thin and aggressive for your car’s engine. In this case I’d recommend looking at the specialist classic car engine oils produced by companies, such as Miller or Comma.

Find out how to hibernate your classic car for winter

How to check your oil

The chances are that your dipstick and oil filler cap are yellow, making them easy to find. If it isn’t obvious, then your car’s handbook will show you where they are.

To check your oil:

• Make sure you have the recommended engine oil to hand.

• Turn your engine off and let it stand for five minutes or more to let the oil drain into the bottom of the engine. Be careful if the engine has been running as everything under the bonnet – including the oil – will be scalding hot. This is why I check my oil first thing in the morning when the car is cold.

• Pull out the dipstick. Wipe it on a lint-free cloth or piece of kitchen roll.

• Push the dipstick back in and make sure it is fully seated. Withdraw and look at where the oil level is.

• There are two marks on your dipstick and the gap between the two is one litre. This helps you gauge whether you need to add a whole one-litre bottle, or just a small part of it.

• If you need more oil then undo the oil filler cap. Top up the oil carefully, making sure not to spill any on the engine. (If you do spill some, wipe it up; it won’t harm your engine but might make a small amount of smoke when it burns off!)

• Give it a minute or so to drain to the bottom of the engine. Wipe the dipstick and re-check the oil level.

• Repeat until your oil is level with the ‘full’ mark on your dipstick.

• Replace the dipstick and oil filler cap.

That’s all there is to it! If you have any questions, please leave them in the comments section and we’ll do our best to answer them.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.