Engine oil and your DPF: why low-ash matters

The wrong oil will not stop a diesel running. It will quietly fill the particulate filter with ash that never burns off.

What ash actually does

Soot burns off during a regeneration cycle. Ash does not. It comes from the metallic additives in the oil, passes through the combustion chamber with the small amount of oil every engine consumes, and settles permanently in the filter substrate. Every litre of high-ash oil consumed brings a filter replacement closer.

How the C classes limit it

ACEA's C sequences cap sulphated ash, phosphorus and sulphur. C2 and C3 are the classes most European diesels specify. An A3/B4 oil may be an excellent lubricant and still be entirely wrong for a DPF car, which is why the class letter matters more than the price.

Short journeys make it worse

A regeneration cycle needs sustained exhaust temperature. Cars that never complete one accumulate soot as well as ash, and the extra fuel dumped into the cylinders during attempted regens dilutes the oil. This is the combination that ends in a blocked filter at 60,000 miles.

Signs the filter is loading up

More frequent regeneration cycles, a faint smell during them, rising oil level between services and eventually a warning light. If the oil level is climbing rather than falling, fuel is getting into the sump and the oil should be changed regardless of mileage.