Engine Oil Explained – Weights, Additives, Brands

Oil is far more complicated than I’d care to admit, but it’s something every car owner should have at least some understanding of, so let’s break it down. The first thing might be the most obvious, where does it go and what’s it even for?

Well, hopefully that is actually obvious to you, but the answer is in here – your oil fill cap. Unless you are doing an oil change, you shouldn’t need to use this much, but it’s there if you need it. As for what it’s for, again hopefully it’s obvious, as it does a rather good job of lubricating all the spinning and sliding metal inside your engine from destroying itself as you scream down the motorway. 

But that’s not all! The less common, but equally important other features are cooling and cleaning. Especially for your pistons – the big lumps of metal dancing up and down inside the engine taking the full brunt of the explosions happening just above them. Pistons don’t get active cooling like the block. There’s no coolant flowing through them, so to keep them cool your engine generally has oil squirters that shoot oil up at the bottom of the piston which helps wick heat away. On top of that, the oil is designed to collect any extra particulates left from those explosions or little bits of metal that have worn away from the bearings, piston rings or cylinder walls. Without that property, your engine wouldn’t last long at all as those tiny bits of crud would scrape up a storm everywhere.

So that’s where it goes and what it does – but when you walk into any parts shop you’ll notice a hundred different options all with slight differences in their labelling, so what does that all mean? Let’s start with the big one, the weights. 5W-30, 5W-40, 10W-30, 0W-40. These are their cold and warm weight – thickness, runiness – ratings. The higher the number, the thicker the oil is. The first number – 5W, 10W, 0W – that’s the cold weight. When the oil is cold, it’s a “5 weight”, or a “10 weight” oil. The second number is its warm weight, when it’s fully up to temperature, so it’s a “30 weight” or “40 weight” oil when warm. 

This is a little counter intuitive. When things heat up they tend to get looser, liquids get thinner, but oil is the opposite. The gets thicker when heated thanks to some of the special additives I’ll cover in a second. The reason for this is important though. When your engine is cold all the metal is much tighter together, all the tolerances are much smaller, so you want your oil to be thinner to be able to be forced between all those tight gaps. But once your engine heats up, all the metal inside expands leaving larger gaps between all the moving components. That means the oil needs to be thicker to stay in those larger gaps and to build more pressure.

What oil your car needs is normally listed in your owners manual, or is often only a quick Google search away. It’s worth noting that what weight oil you use is determined by where you live. Here in the UK you’ll find a lot of 5W-30 as it’s better at lower temperatures, whereas in warmer regions you might find 10W-40 or 15W-40 is more popular. The weight of oil isn’t a hard and fast rule, if you have no other options and not putting any in is going to leave you stranded then don’t worry about it, although following the recommendations is, well, recommended.

Now I mentioned additives earlier, this is often called the “additive package” and it’s chock full of chemicals that add an impressive suite of features. This generally makes up around 20% of your oil, with the other 80% being the “base oil”. The base oil is generally made of their mineral or synthetic oil, with mineral oil being refined crude petroleum whereas synthetic is a man-made stock. Generally synthetic is more pure, although some especially older cars might prefer the cheaper mineral oil instead. 

As for the additive pack, that contains those viscosity modifiers I mentioned that help it get thicker at higher temperatures, as well as detergents to keep your engine clean, friction modifiers to make your engine parts move more freely, oxidation inhibitors to maintain its lifespan and anti-foaming agents so it remains a whole liquid and can be pumped at high pressure effectively.

These additives, along with the base oil, are generally formulated to meet a set standard. This is often harder to see even on the bottles themselves, but they will be there. These standards don’t tie the weights in, so you can buy say a 5W-30 that meets the same spec as a 0W-40. There are a couple of groups of standards at least here in Europe, there are the ACEA standards and the ones from the car manufacturers. 

ACEA has a fair few, but the ones you’ll need to know about are: A for petrol; B for diesel; and C for vehicles with particulate filters or catalytic converters – which these days is most if not all cars. C spec oils will produce less soot and SAPS – sulphur ash, phosphorus and sulphur – meaning the fine mesh in your DPF, PPF or Cat won’t get clogged as quickly. You’ll see this listed as something like A3/B3, A3/B4, C3 or C4, with the numbers I think being revisions with the higher the newer/better. 

The manufacturer specs can vary between brands, VW has their range of “500.00”, like 502.00, 505.01 or 507.00. BMW has “LongLife” specs like LL-04, LL-14+ or LL-17 FE. And Ford… their ones are pretty terrible so I’ll just stick some of them on the screen. Each auto-maker often has their own specs – many of which are largely the same as others or based on the ACEA specs. Getting an oil that meets the right spec for your engine is important if you want to take care of your car. If you are in a pinch or can’t find the right one it’s not the end of the world – you are generally trading convenience for lifespan by using whatever you can find. 

So you know what weight and spec your oil should meet – great! You can walk into the parts shop confident you can get the right… oh. There’s like 20 different brands all selling options that supposedly meet your needs… Which of those do you choose? Well, for the most part, and in theory, the brand doesn’t matter much. If it meets the weight and spec you need, you should be good to go. Especially if you don’t have a high performance engine, you can generally go with whatever oil you fancy, including the cheaper options. You might see on your oil fill cap that the manufacturer has done a brand deal with an oil supplier – it might say something like “Works best with CASTROL oil”. As a general rule, this is marketing BS that you can ignore.

If you have a higher performance engine, or you are one of the very, very vocal oil snobs, then it’s often a good idea to see what other owners recommend, or pay attention to the specific added features. For example LiquiMoly, a brand I personally quite like and use, sell oils with Molybdenum Disulphide which is a great low-friction additive, which might be a good benefit over say a Castrol Edge that doesn’t include that. Some engines can be picky – I’ve heard stories of engines having terrible oil consumption – ie burning the oil during combustion which is bad – using one brand, then switching to a different brand (all the same weight and spec) and the consumption goes away. That’s just from the rumour mill though.

The final thing to mention is the service life. Many oils like to quote “extended oil change intervals”, like the BMW LongLife spec, but this is generally a bad idea. Your oil degrades over time and with use – the residue from combustion turns your oil acidic and the oil itself – despite your oil filter’s best efforts – gets saturated with soot and debris. That’s why when you see clean oil going in it’s often nice and golden, and when you drain your used oil out it’s black. It’s full of particulate. As a generalised, sweeping rule, doing an oil change once per year or every 10,000 miles is what most manufacturers recommend. Some are pushing for 15,000 or 20,000 miles, but that’s a pretty bad idea. If you want it to last, do it at least every 10,000 or once per year.