5W-50 or 0W-30?

It might seem that everything about the viscosity of engine oils has already been chewed over, but apparently not quite. The questions that keep coming up on the site’s forum suggest there’s still more to say on the subject of oil viscosity. So, which is better to choose, a higher or a lower engine oil viscosity? And what should you do if the warranty service fills your car with an oil whose viscosity isn’t the one specified in the owner’s manual?

Let me say it once again straight away: your car’s oil viscosity has to match the manufacturer’s requirements, regardless of the car’s age, mileage, driving style, budget, or the “authoritative” opinion of service technicians, even at an official dealer. This article is written for those who have doubts, and for those who are simply curious why that is. If that’s you, read on; if not, read the owner’s manual (or the service book), and insist that you’re only ever given the engine oil specified by the engine’s designers, in every respect, viscosity included.

So, let’s dig into the question of engine oil viscosity. The friction pair in the engine that’s easiest for most drivers to picture is the “piston-cylinder” pair, so for the sake of clarity we’ll use that very pair in our little logical exercise.

The piston with its rings doesn’t rub against the cylinder wall metal on metal: between them there is an oil film only a few micrometres thick. Over most of the stroke the piston literally glides on it, like a water-skier on water: the higher the speed and the oil’s viscosity, the thicker the film. But at top and bottom dead centre the piston stops in order to reverse, and its speed falls to zero. Near top dead centre the combustion pressure and the highest temperature are added to that. This is where the film is thinnest, and this is exactly where, at the level of the top compression ring, the cylinder wears most. The same laws apply in the crankshaft bearing shells, where the film is held in place by the rotation of the journal, and in the cam-and-tappet pair, where the load is so great that viscosity alone can’t save the day and the main work is taken over by the anti-wear additives.

The designer chooses the clearances, the materials and the capacity of the oil pump for one quite specific viscosity at all of these points. Let’s see what happens if you change it in one direction or the other.

If the oil is thicker than it should be

On a warmed-up engine under gentle driving, no great harm is done: the film is thicker, and the rings and bearing shells are protected with a margin to spare. But that margin has to be paid for. Thick oil is harder to push through the passages and to shear in the clearances, so the engine spends more energy on its own friction and fuel consumption goes up. The difference is small, but it is always there. What is more, oil doesn’t only lubricate, it also cools: thick oil passes through a bearing clearance more slowly, so it carries away less heat, and the viscous film itself heats up more. I have already explained why an engine on too-high viscosity constantly runs hotter than it should in the article on too-high oil viscosity.

Thick oil does most harm on a cold start. The gauge may even show a higher pressure than usual, but that is the pressure at the pump: the excess is dumped back by the relief valve, while the thick oil reaches the camshafts, the hydraulic tappets and the turbocharger bearing noticeably later. During those first seconds the engine runs almost without lubrication, and they cost it the most service life (there is a separate article on the loads during warm-up).

But the common fear that thick oil is burned off more through the oil control rings is, for the most part, unfounded. As a rule the opposite is true: thicker oil seeps past the rings and the valve stem seals less readily, so consumption drops. That is exactly why it is so willingly poured into worn engines, and exactly why this is misleading: the problem doesn’t go away, it just hides (this is covered in popular misconceptions about viscosity).

If the oil is thinner than it should be

Here the picture is the reverse, and it is more dangerous. In a hot engine at high revs and under load, thin oil gives a thinner film exactly where it is already at its limit: near top dead centre, in the big-end bearings, in the turbocharger bearing. The film breaks more often, metal touches metal, and wear grows. Thinner oil also passes the rings and valve stem seals more easily, so you will have to top it up more often. And at a hot idle, especially in a high-mileage engine, the pressure can fall so far that the oil pressure warning light starts to flicker. That is no longer “wear sometime later”, it is a signal that the loaded components are short of film right now. Oils with a reduced high-temperature viscosity, which are made only for special engines, are covered separately in the article on too-low viscosity.

A modern engine on 5W-50 and an old one on 0W-30

Now for specific cases. Take a modern engine for which the carmaker specified 0W-30, and fill it with 5W-50. The number before the W hardly decides anything here: in a typical European winter, 5W and 0W oils start the engine equally confidently. The difference is in the second number: in a warmed-up engine, a grade 50 oil is noticeably thicker than a “thirty”. Such an engine most likely won’t break tomorrow, or in a year. But it has thin oil passages, a turbocharger, hydraulic tappets, a chain with a hydraulic tensioner and variable valve timing phasers that are controlled by oil pressure and designed for a particular viscosity. On thick oil the phasers react more slowly, the engine control unit may log a fault, fuel consumption goes up, and every cold start costs the engine more of its service life.

Now the other way round: an old engine designed for 10W-40 or even 5W-50, with larger clearances and a pump built for thick oil, gets 0W-30. In winter it starts more easily, and that is where the advantages end. In summer on the motorway the oil pressure is lower, the film in the bearing shells is thinner than the designer intended, oil consumption is higher, and the pressure light may flicker at a hot idle. It is even worse if it is a 0W-30 with a reduced high-temperature viscosity (ACEA class A5/B5): for such an engine that is a direct threat to the bearing shells.

Which is worse

These two deviations are not symmetrical. For an old engine designed for thick oil, too-low viscosity is more dangerous: the consequences arrive faster and can end in a spun bearing. For a modern engine designed for thin oil, too-high viscosity does harm more slowly and less visibly, but not with impunity either: the cold start suffers, as do the systems that run on oil pressure, and so does fuel consumption. There is one exception too: in severe frost the most dangerous viscosity is the too-high one, only the low-temperature kind, that is, too large a number before the W.

So the answer to the question in the title is simple: neither 5W-50 nor 0W-30, but the viscosity the carmaker specified for your engine. If it gives several options, choose among them, and that is the whole room for manoeuvre you have.