A Guide to Lubrication For Your Diesel Engine

February 21, 2017 - 9:00 AM by Samantha Pudney


How much do you know about the necessary lubrication for your diesel engine? Despite this being basic engine knowledge, many skippers only understand the tip of the iceberg. First and foremost, lubricating oil acts as a barrier between moving parts, reducing wear in your diesel engine.

Normally, it is distributed around the engine by means of a pump before being returned to the sump. In some slow running engines, the lubrication will be different. Instead, oil is thrown to the bearings, bores etc. as the engine turns over.

In a pumped system, the oil normally flows to the bearings, bores etc. through the holes either cast or drilled into the engine casing. It’s crucial that you keep the lubricating oil in the engine within the limits marked on the dipstick!


Constant displacement pumps are driven continuously from the engine. Their output will depend on your engine speed. The internal parts of the pump cater to fine tolerances and as such, use very close fitting parts.

Any undue wear will harm their efficiency. This will become clear to you if the pressure generated at a low speed is not enough to operate the pressure switch. Or, alternatively, if it fails to meet the recommended minimum pressure of the manufacturer.

Suction Strainer

A suction strainer is fitted primarily to protect the pump from sucking in any misplaced nuts and bolts. The strainer is low down in the sump and connected to the pump by a durable steel pipe.

Relief Valve

The oil pressure in your diesel engine is determined by the resistance to flow and limited by a relief valve (which is factory-set). This is a spring-loaded plunger which raises to allow oil to pass directly from the outlet of the pump to the inlet. Or, to the sump when the setting is reached.

If debris is able to jam the relief valve open due to poor system maintenance, then the oil will bypass continuously and the working pressure will not be achieved.


Oil Filter

Your oil filter ensures that the oil reaching the moving parts is clean. The quality of filtration is determined by the engine manufacturer.

Filter cartridges may incorporate reverse flow non-return valves and a bypass. This is set to operate at a specific pressure, in case the filter becomes blocked. Note: only the manufacturer’s recommended spares should be used in this instance.

The filter element should always be changed when you change the lubricating oil, which also makes it easy to remember. Before restarting your diesel engine, turn it over by hand or the starter motor, with the fuel pump engine stop valve closed. This will fill the filter and passages.

Lubricating Oil

The oil used for engine lubrication is a sophisticated product. Some diesel engines require a higher specification oil than that used for petrol engines. The reasons are this:

  • The working pressure is higher in diesel engines
  • The running temperature can be raised
  • Chemical conditions created by the combustion system are more severe

There are various systems of classifying lubricating oil. Oils to a higher classification are generally suitable for applications of a lower class. A synthetic oil of a high grade is needed to provide the correct lubrication for a turbo-assisted engine.

Note: always refer to the engine manufacturer’s instructions for both viscosity grades and the minimum quality required.


Mixing oils

Diesel engine lubricating oil is designed to be mixable with oils of a different standard. Engines cover a wide range of service conditions from low powered to heavily loaded applications. This means that the more taxing your service is, the higher the specification of oil required.

Naturally aspirated engines, particularly those operating in lightly loaded applications, may need a lower grade oil for running in, to bed in the bores and avoid bore polishing which can result in high lubricating oil consumption.

Note: always consult the manufacturer for recommendations when running in a new or reconditioned engine.

Where a high sulphur content fuel is used, your lubricating oil should be changed more frequently. Often, oil companies will provide an oil analysis service. This is helpful to determine wear rates and combustion efficiency.

Planning a long trip this year? Take at least enough oil and spare filters for as many changes you expect to make. To avoid extra stress, ensure that you plan to exceed the recommended change intervals. This means if something happens, you won’t need to use an oil that you don’t know much about.


It’s best practice to avoid direct skin contact with any mineral oils. Used diesel oil has deposits in it formed during combustion which could be potentially harmful to your health. Always wear protective gloves as an added precaution while you change the oil or filters.