Engine oils: why using the wrong one can prove catastrophic

The sheer number of automotive engine oils on the market can easily cause a headache for technicians.

This won’t be helped by a whole lot of new references that need a super-thin 0W-20 oil, but crucially not every engine used in the same vehicle family uses the same viscosity of oil.

And that’s before we get in to the agony of the correct ACEA spec and manufacturer’s approval numbers, where getting either of these wrong can lead to warranty rejection if anything goes wrong – even if it has nothing to do with the oil.

Four-step process

There are a couple of ways to stop making a mistake though.

The first, and easiest way is to enter the vehicle number plate into one of the lube checkers on the blenders’ websites.

The other way of nailing the essential information without a degree in oil engineering is to use the ‘four-step’ method.

Step one: How is the oil made?

Unless you’ve wandered into the ‘classic oils’ section of your local accessory shop, or you are considering purchasing house-branded oil from Tesco, it will always be fully synthetic.

Step two: What is the viscosity?

Find out what multigrade viscosity is required, for example 0W-20, 5W-30 etc.

Step three: What is the ACEA rating?

In some instances there will only be an ILSAC or API specification – be sure to get this right because we are in the warranty-rejection zone.

Step four: What is the manufacturer’s approval number?

Tread carefully here. Newer approvals such as VW 508.00/509.00 are not backwards-compatible with 504.00/507.00, and using the wrong one will mean warranty rejection.

Interestingly, the new spec oil is green in colour although this has nothing to do with its lubricating properties. Why do engines need such specific oil these days?

It’s likely that modern engines are exquisitely designed with almost impossibly tiny tolerances and place very specific demands on engine oil and the additives are designed specifically for an engine. There can be real-world consequences for using an out-of-spec oil.

What are the key consequences?

One of these consequences is low-speed pre-ignition.

LSPI is a new phenomenon, occurring in modern Gasoline Direct Injection (GDI) engines, although some of us may remember this occurrence from back in the day, as pre-detonation or pinking.

With LSPI the resultant detonation can have disastrous consequences such as putting a hole in a piston.

However, the cause of LSPI can be traced back to certain additives used in the engine oil. The new generation of ILSAC GF6-A oil has specific additives to combat this.

The short answer here is don’t risk using the wrong oil, even if the vehicle is out of warranty and the customer is pleading poverty.

It isn’t worth it, especially as there are plenty of tools available to make sure that the correct product is used.

One car, many oils

I took a look at the owner’s manual of a 2019 Mercedes A class petrol to see what oil it needed. There were two big surprises there.

It gives no recommended viscosities, just the Mercedes-Benz specification. Secondly, there are four MB specifications for the 1.3L petrol engine, and eight for the 1.9-litre diesel.

This is tricky for anyone who needs to make sense of historic, current and future oil specs. The key take is that Mercedes Benz is 100% focused on only the MB level-four specifications (eg MB 229.71 and the level-2 viscosities, by default, fit into the correct MB level-four spec).

But this is not easy when there are 11 5W-30 grades and a growing number of 0W-20 grades… Oh, and throw in 0W-30, 0W-40 and 5W-40 grades that may or may not have the correct MB specification.

By Simon Michell, oil expert

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