(i) Oxidation: The chemical reaction of the lubricating oil with oxygen (at high temperature) that breaks down the oil, forming oxidation products (acids, sludge, varnish, and increased viscosity). Oxidation degrades the oil's lubricating properties.

(ii) Emulsification: The formation of a stable mixture of oil and water (a milky emulsion), caused by water contamination and agitation. Emulsified oil has poor lubricating properties and can cause corrosion.

(iii) Acidity: The acid content of the oil, measured by the Total Acid Number (TAN). Acidity increases as the oil oxidises or becomes contaminated (e.g. with combustion products/ acids), and high acidity causes corrosion of the engine components.

• Oxidation: Controlled by maintaining the oil temperature within limits (avoiding overheating), by using the correct oil with oxidation inhibitors, by regular oil changes/ top-up, and by removing oxidation products (sludge) through filtration/ purification (centrifuge).
• Emulsification: Controlled by preventing water ingress (maintaining the cooling/ sealing systems), by removing water from the oil (centrifuge/ purification, settling), and by regular oil testing/ draining of water.
• Acidity: Controlled by regular oil changes/ top-up (diluting the acid), by using the correct oil with the right TBN (for diesel engines, to neutralise acids), by removing acids through purification, and by monitoring the TAN/ TBN and changing the oil when the acidity exceeds the limit.

• Oxidation: Increased viscosity and sludge/ varnish formation, blocking oil ways and filters, reduced lubrication, increased wear, and possible bearing/ engine failure.
• Emulsification: Poor lubrication, corrosion of the engine components, reduced oil film strength, and increased wear/ failure.
• Acidity: Corrosion of the bearings, journals, and other components, pitting, and accelerated wear; the oil loses its protective properties, leading to engine damage.
• In all cases, failure to correct the condition leads to accelerated wear, damage, and possible catastrophic engine failure.