Which oil goes into a diesel with a particulate filter?

With a particulate filter, everything seems clear enough: the warning light comes on, and the culprit is the fuel, the traffic jams or the filter itself, which was “invented to squeeze money out of people”. The oil is the last thing anyone thinks of. And wrongly so: the filter can burn off soot on its own, but not the ash from the oil.

Why a diesel has a filter and what collects in it

The first modern particulate filters (DPF; on French cars, FAP) appeared as early as 2000, and by the mid-2000s many diesels already had one. It was Euro 5 that made them universal: it cut the permitted mass of particulates in a diesel car’s exhaust fivefold, from 0.025 to 0.005 g/km, and later limited the number of particles as well. Those figures cannot be met without a filter, so ever since, practically every diesel car sold in the EU has had one.

Soot is unburnt fuel, and from time to time the filter burns it off. But every engine burns a little oil, and the oil contains additives with zinc, calcium and magnesium. When they burn, they leave incombustible ash (why it cannot be burnt away, any more than the ash in a stove can, is explained in the article on ACEA classes). It builds up in the filter after every regeneration, raises the exhaust back pressure and makes the filter regenerate more often.

How much ash ends up in the filter depends on two things: how much oil the engine burns and how much ash there is in the oil itself. The second is down to your choice at the shelf. In heavy-duty diesels, where this has been well studied, after about 240,000 km ash makes up more than 80% of what sits in the filter. There are no such figures for cars, but the mechanism is the same.

Then there is phosphorus from the anti-wear additive: it does not clog the filter, but it poisons the catalytic converter, which then only starts working at a noticeably higher temperature. Hence the abbreviation SAPS: sulphated ash, phosphorus and sulphur, the three things a “filter-friendly” oil must contain little of.

What to look for on the can

ACEA class: C only

For cars with a particulate filter ACEA (the European Automobile Manufacturers’ Association) has set aside a family of its own, the C classes, which, in its own words, extend the life of the filter and the catalytic converter. The difference from ordinary A/B oils lies in the figures (ACEA 2023 edition):

In other words, an A3/B4 oil may contain twice as much ash as C3 and more than three times as much as C4. And do not be put off by “mid SAPS” on the label instead of “low SAPS”: both are oils for the filter.

Within the C family the classes also differ in high-temperature viscosity: C5-C7 are thinner than C3 and C4 and suit only the engines designed for them.

The manufacturer’s approval matters more

Let me say it once again: the class is only the base, and the manufacturer’s approval adds its own tests on top of it (see what oil approvals are). If the service book specifies an approval, look for exactly that one on the can. For diesels with a filter these are, for example:

1. Volkswagen, Audi, Skoda, Seat: VW 507 00, an oil based on ACEA C3, as a rule 5W-30 (see the VAG approvals explained). Some newer engines require VW 509 00 (0W-20), and the one cannot be swapped for the other.

2. Mercedes-Benz: 229.31, 229.51, 229.52, and in most newer engines 229.71. The full-ash 229.5 and 229.6 are no good where a low-ash approval is required.

3. BMW: Longlife-04; for some single-turbo diesels, LL-12 FE, later joined by LL-19 FE.

4. Opel: dexos2, which GM has replaced with dexosD; newer Opels with Stellantis diesels follow Stellantis approvals.

5. Peugeot and Citroën: PSA B71 2290 for HDi diesels with FAP; on newer cars, Stellantis FPW 9.55535.

6. Ford: WSS-M2C934-A/B; on newer diesels, 950-A or 952-A1.

7. Renault and Dacia: RN0720; the newer RN17 does not replace it.

8. Japanese makes: for Japanese-market cars, JASO DL-1, a low-ash class for diesel cars with a filter; for European versions check the book, which usually calls for an ACEA C class.

This list is a guide, not a substitute for the service book; more in the article on carmakers’ new approvals.

Short trips, regeneration and fuel in the oil

How the filter cleans itself

On a long, fast road the exhaust is hot (roughly 350-500 °C), and the soot burns off by itself: this is passive regeneration. When the filter fills up, though, the engine control unit sets off an active one: it adds a late fuel injection to raise the exhaust temperature. According to an estimate by the British motoring organisation the AA, this happens roughly every 500 km and takes 5-10 minutes.

Now picture a car that does a few kilometres at a time around town: the regeneration has started, but you have already arrived and switched off the engine, like a pie taken out of the oven half-baked every single time. Frequent short trips are the most common cause of a clogged particulate filter. If the warning light comes on, ten minutes or so of driving at over 65 km/h usually helps (the owner’s manual gives the details). Ignore it, and the engine goes into limp-home mode, followed by a forced regeneration at a garage or, at worst, a new filter. As for the miracle bottles “for cleaning the filter”, I trust them no more than any other additives: a filter is cleaned by driving, not by chemistry from a shop.

Why the dipstick shows more oil

Late injection has a side effect: some of the fuel runs down the cylinder walls into the sump. Biodiesel, of which diesel fuel may contain up to 7%, does not evaporate from the sump and builds up, and every interrupted regeneration adds a little more. How this affects the change interval is explained in the article on oil change intervals today.

It has come to the point where the dipstick of some Mazda diesels carries an extra “X” mark above the maximum: once the level reaches it, there is too much fuel in the oil and the oil must be changed immediately. And apart from driving style, a filter is clogged by high oil consumption, full-ash oil, a sticking EGR valve and faulty injectors.

What has AdBlue got to do with it?

AdBlue (generic name AUS 32) is a 32.5% solution of urea in demineralised water to ISO 22241. It goes into a separate tank for the SCR catalyst, which reduces nitrogen oxide emissions. AdBlue has nothing to do with oil or fuel, and it goes neither into the fuel tank nor into the engine. The only link between them is this: catalysts like neither phosphorus nor ash, so the right oil protects the whole exhaust after-treatment chain, not just the filter.

A “deleted” filter

A separate subject is a filter that has been cut out, with the control unit reflashed. There will be no details here. Under Directive 2014/45/EU, missing or clearly defective original emission control equipment is a major defect, and in EU countries the car fails its roadworthiness test. In Germany, since 1 July 2023, the periodic inspection of Euro 6 diesels counts the number of particles in the exhaust, and without a working filter there is no staying within the limit. In Ukraine, the Rules of the Road (clause 31.4.6) prohibit operating a car if the harmful substances in its exhaust exceed the limits or the exhaust system is faulty. And without a filter the oil requirements do not change: put in what the service book says.

What does this mean for the ordinary car owner?

1. The service book first: it, not the year of manufacture, will tell you whether your car has a filter; then comes the manufacturer’s approval, then the class. Do not pour an A/B oil in where C is required, even if it is cheaper.

2. Do not “improve” the oil with a thinner one. C5-C7 or 0W-20 only if they are specified for your engine.

3. Give the filter a chance to clean itself: get out onto the motorway regularly.

4. Keep an eye on the dipstick. A level that rises between changes is a reason to change the oil now, and town driving is a reason to shorten the interval.

And what if you had to top up with a litre of ordinary A3/B4 on the road, you ask? Most likely nothing terrible: the harm comes not from one top-up but from change after change with the wrong oil.

So, a particulate filter lives as long as two things allow: oil with the right approval, and trips long enough for it to clean itself. Nobody can honestly tell you how many kilometres a full-ash oil takes off its life. But the price of the right can bears no comparison with the price of a new filter, so testing it on your own car is very much a false economy.