Which motor oil for a diesel?

The particulate filter dictates the choice of oil

Some of the motor oil gets past the piston rings into the combustion chamber and burns with the fuel. Whatever ash-forming components remain travel into the diesel particulate filter and settle there for good. Soot can be burnt off during regeneration; ash cannot. That is why manufacturers specify low-ash oils for vehicles with a particulate filter, known in the trade as Low SAPS or Mid SAPS. SAPS stands for Sulphated Ash, Phosphorus, Sulphur. A conventional oil with a high ash content clogs the filter noticeably faster, and a blocked particulate filter costs many times what was saved on the oil.

ACEA C1, C2, C3 and C5: the difference

The ACEA C classes are the classes for vehicles with exhaust aftertreatment. They differ in two respects: the permitted share of ash, phosphorus and sulphur, and the HTHS viscosity, meaning how thick the oil stays at high temperature and under high shear.

• C1 is the lowest-ash class and sits at the lower end for HTHS viscosity. It was specified mainly by Ford.
• C2 allows a little more ash and stays at a low HTHS viscosity. Typical for many French engines.
• C3 has a markedly higher HTHS viscosity of at least 3.5 mPa·s with a medium ash content and is by far the most common requirement in Germany. You will find the matching oils under ACEA C3.
• C5 lowers the HTHS viscosity below even the C2 level and serves to reduce fuel consumption. It is only permitted for engines expressly designed for it.

This is exactly where the most expensive mistake is made: anyone who pours a C1 or C5 oil into an engine specified for C3 is running a thinner lubricating film under high load than the manufacturer intended. Conversely, a C3 oil in an engine designed for C5 can raise fuel consumption and, in some cases, affect the variable valve timing. The classes are not ranked by quality but sorted by engine design. A higher number is not a better oil.

The manufacturer approval ranks above the ACEA class

The ACEA class is the baseline requirement. Above it sit the vehicle manufacturers' approvals, which demand additional engine tests of their own. For diesels in Germany these are above all VW 507 00 for vehicles with a particulate filter, VW 505 01 for the older pump-injector engines, MB approvals 229.51 and 229.52, BMW Longlife-04, and the requirements of Opel, Ford, PSA and Renault. If the handbook names an approval like this, it is binding: an oil that only meets the ACEA class is formally not the prescribed oil for that engine, even if it is technically similar in make-up. What the individual designations mean is explained in the article Reading manufacturer approvals and specifications.

Diesels without a particulate filter

On older diesels without exhaust aftertreatment there is no filter for ash to clog. These engines were mostly approved for ACEA A3/B4 oils, often in 5W-40 or 10W-40. A low-ash C oil is not forbidden there, but it brings no advantage either, and its often lower HTHS viscosity does not suit an engine designed for a thicker oil. What counts is the requirement in the handbook, not the year of manufacture.

Viscosity in winter and when towing

The first number of the viscosity grade describes cold-start behaviour, the second the thickness at operating temperature. A 5W-30 reaches all the lubrication points faster on a cold start than a 10W-40, and on a diesel with its high injection pressures in particular, every second without full oil pressure counts. Even so, anyone who tows a lot or often drives heavily loaded should not switch to a thicker oil on their own initiative: the manufacturer's approval already allows for these load cases, and a deviation may well cost you your warranty claim. You will find the common pack sizes under 5W-30.

Fuel dilution and short trips

Diesels with a particulate filter inject extra fuel for regeneration. If short trips keep interrupting the regeneration, unburnt diesel gets into the motor oil, dilutes it and reduces its lubricating ability. A rising oil level on the dipstick is the visible sign. In this case a different oil is no help; what helps is a shorter change interval and the occasional longer drive during which the regeneration can run to completion.

How to find the right oil

The quickest route starts from the vehicle rather than the container: choose make, model and engine in the oil finder, and the list shows only the oils approved for that vehicle. If you are doing the filter change at the same time, the matching parts are in the Filters category.