Keep the oil level between the marks on the dipstick. The colour of the lubricating oil in the system says much for the engine’s health and can be analysed for evidence of internal wear. Diesel lubricating oil becomes black through the action of its additives. Grey means water is present.
The oil used for lubrication is a sophisticated product, and some diesels require a higher specification oil than that used for petrol engines. The reasons for this are:
1. Working pressures are higher in diesel engines.
2. Running temperatures can be higher.
3. Chemical conditions imposed by the combustion process are more severe.
In an oil developed for diesel engines the basic oil content is about 75 per cent, with additives making up the balance. These include:
1. Detergents to keep the inside of the engine clean.
2. Anti-wear additives to extend the life of wearing parts, particularly the valve gear.
3. Oxidation inhibitors which reduce the rate at which the oil base deteriorates when used.
4. Anti foam agents.
5. Viscosity index improvers to keep the oil fluid at low temperatures and thick at high temperatures.
6. Alkaline additives to counteract acidic compounds formed by the diesel combustion process.
7. Anti rust agents for engine protection.
There are various systems of classifying lubricating oils for viscosity. The common ones are
SAE (US Society of Automotive Engineers). A typical specification would be either SAE
10W30 for low temperature applications or SAE 15W40 for summer temperatures.
The standard method of specifying the quality of diesel engine lubricating oils is the API (American Petroleum Institute) reference CA to CE, API CE being the highest quality. The SA to SG range is for petrol engines.
European quality standards are defined by the CCMC (Comité des Constructeurs d’Automobiles du Marché Commun), a typical European engine oil being D4 or D5, or for passenger cars PD2. D5 is for Super High Performance Diesels.