The oil pump draws lubricant from the sump and sends it through the filter and internal galleries to bearings, valve gear, piston cooling jets and turbocharger where fitted. Oil separates moving surfaces, carries heat and contamination, controls deposits and protects against corrosion.
Use the viscosity, performance classification and change interval specified for the exact engine and climate. A modern low-emission engine may require an oil formulation different from an older mechanical diesel. More expensive or thicker oil is not automatically better.
What oil can tell you
- Level rising: possible fuel dilution, coolant contamination, seawater ingress or an incorrect reading procedure.
- Level falling: external leak, burning oil or oil moving into another system.
- Milky oil: water contamination. Stop and investigate.
- Strong diesel smell: possible fuel dilution.
- Metallic particles: abnormal wear requiring prompt assessment.
- Sudden pressure loss: stop the engine as soon as safely possible. A failed sender is possible, but continued operation can destroy the engine.
Take repeatable dipstick readings with the boat in a comparable attitude and using the manual’s hot or cold procedure. Overfilling can cause foaming, crankcase pressure and oil ingestion. Underfilling risks loss of pressure when the boat heels or pitches.
An oil analysis program can trend wear metals, soot, viscosity, fuel dilution and coolant markers. A single result is useful; a consistent sampling history is far better.
Related: Changing oil and filters · Low oil pressure alarms