Once again about oil viscosity
On oil viscosity being too high or too low
In this section: 7 articles
On the site’s forum, the same basic question comes up again and again: which viscosity of automotive oil is better to choose for different engines, and why official service centres sometimes fill something other than what’s written in the owner’s manual. Given how often these questions come up, it seemed to me worth returning to the topic of automotive oil viscosity once more.
Only this time let’s approach it from the opposite direction and ask the question differently: what’s worse for the engine — engine oil viscosity that’s too high, or too low? And what happens inside the engine when the viscosity doesn’t match the car manufacturer’s requirements?
Viscosity is often argued about as if the engine ran in a single mode all day. In reality, over a day the oil travels from a frosty parking space to a hot motorway, and in each mode “bad” means something different. So let’s go through them one by one.
From a frosty morning to a summer motorway
Cold start at −20 °C. Here the number before the W decides everything. Cold oil is hundreds of times thicker than in a warmed-up engine, and the starter has to turn the crankshaft while the pump pushes the oil through the passages to the most remote components. At −20 °C, 0W, 5W and 10W oils will cope, while 15W is already at its limit. The second number plays no part at this stage. How to read both numbers is explained in the article on the viscosity index.
Warm-up. The coolant heats up faster than the oil: the temperature gauge is already at the middle of the scale, but the oil in the sump is still noticeably colder and thicker than at working temperature. So for the first few kilometres it is better to drive gently, without high revs (more on warm-up).
Town. Short trips and traffic jams are the most insidious mode. The oil often doesn’t get properly warm, condensate collects in it, and in direct-injection petrol engines so does fuel, which thins it. In other words, in real-world use the oil often becomes thinner than what’s written on the can. This is one more argument against going below the carmaker’s requirements: the designer has already allowed for the thinning, and the town will eat up your own “margin downward” all by itself.
Motorway under load and a hot day. A long drive at speed with a full load, a long uphill haul with a trailer, or a traffic jam at +35 °C with the air conditioning on — and the oil gets hottest of all. In the bearing shells, near the piston rings and in the turbocharger bearing it is much hotter than in the sump. What works here is no longer the number before the W, but the second number and a figure that isn’t usually printed in large letters on the label.
HTHS: viscosity where it is hardest
HTHS (high temperature, high shear) is the oil’s viscosity at 150 °C and a very high shear rate, that is, in conditions close to those in a loaded bearing shell. Two 5W-30 oils with identical numbers on the can may have different HTHS, and this is where they part ways. Classic ACEA A3/B4, C3 and C4 oils have an HTHS of at least 3.5 mPa·s. The fuel-economy A5/B5 are from 2.9 to 3.5 mPa·s, C2 is at least 2.9, C5 and C6 are from 2.6 to 2.9, and C7 is lower still.
A lower HTHS means less friction and slightly lower fuel consumption, but also a thinner film. That is why such oils are made only for engines in which the designer has matched the bearing shells, clearances, surface finish and pump to this thinner film, and the carmaker has verified all of it by testing. In an engine designed for 3.5 mPa·s, an oil with an HTHS of 2.6 gives nothing away on a cold start, but on a hot motorway there will no longer be any safety margin in the film. Why what matters is the behaviour of viscosity in a hot engine, and not a single number on the label, is explained in the article on oil viscosity at working temperatures.
Why the garage recommends something other than what is in the manual
It happens that an official dealer fills a viscosity other than the one written in your manual, and the reasons aren’t always bad:
- climate and a regional manual: for markets with different climates, the carmaker may give different recommendations, and a manual issued for another country doesn’t necessarily match the one the local dealer works from;
- later service bulletins: the carmaker is entitled to revise its requirements after the model has been released, to allow a new approval or replace an obsolete grade;
- servicing regime: for the same engine, the manufacturer may allow a different approval for a fixed change interval than for an extended Longlife interval;
- simply what’s in stock: sometimes it’s like this, a drum of one oil that is used to service everything.
The first three reasons are entirely legitimate, and the garage will be able to show you a document. The fourth is not. So ask not for words but for the number of the bulletin or the page of the service documentation. If they show it, good; if they answer “we always do it this way”, draw your own conclusions.
How to find out the real requirement of your engine
Start with the owner’s manual or the service book. For European carmakers the main thing there is the approval. For example, VW 504 00/507 00 is, as a rule, 5W-30 with an HTHS of at least 3.5 mPa·s, and MB 229.5 is a full-ash oil with the same limit. If an approval is given, choose the viscosity among the oils with that approval on the label. Other manuals have a table of viscosities by air temperature: the bars show the temperature range in which 5W-30, 10W-40 and so on can be used. You need the one whose bar covers both your winter frosts and the summer heat. The limit values of the SAE grades themselves are collected in the table of viscosity values.
To put it briefly, you can’t choose viscosity for a single mode. The engine lives through frost, traffic jams and motorways alike, and the carmaker chose an oil that withstands them all in this particular engine. Thinner oil lets you down where the load is greatest, thicker oil where the engine is most vulnerable, in the cold and during warm-up. Neither of them is a “safety margin”: the margin has already been built into the requirement the carmaker wrote down.