Section 16.8 / 16.9

Lubricants, Fuels & Lubrication Systems

Mineral vs ashless-dispersant oils and their jobs, wet- and dry-sump systems, and AVGAS grades.

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Aircraft piston-engine oils are either straight MINERAL or ASHLESS-DISPERSANT (AD): mineral oil is used for the break-in of a new/overhauled engine, then AD oil (which holds combustion by-products in suspension for the filter to catch) for normal service.
Oil does six jobs: it lubricates, COOLS, cleans (carries debris to the filter), seals the piston rings, protects against corrosion and cushions shock loads; it is graded by viscosity (SAE/commercial aviation grades).
A WET-SUMP system keeps the oil in the crankcase — simple and light, standard on most light aircraft. A DRY-SUMP system holds oil in a separate tank with scavenge pumps returning it, giving reliable supply at any attitude (aerobatic and larger engines).
The oil system has a pressure pump, filter, oil cooler, pressure relief valve and pressure/temperature indication; the oil pressure gauge is the engine's vital sign.
Aviation gasoline (AVGAS) is graded by anti-knock (octane/performance number); 100LL ('low lead', dyed blue) is the common grade — using too low a grade invites detonation, and grades must not be mixed carelessly.
⚠ Exam trap: a WET-sump keeps oil in the crankcase (light aircraft); a DRY-sump uses a separate tank + scavenge pumps for supply at any attitude (aerobatics); a NEW/overhauled engine is run in on straight MINERAL oil before switching to ashless-dispersant.

Solved examples

Why is a freshly overhauled cylinder run on straight mineral oil rather than ashless-dispersant oil?
During break-in the piston rings must bed into the cylinder wall, which needs a little controlled wear. Straight mineral oil lets the rings seat properly, whereas ashless-dispersant oil's additives can glaze the bore and prevent the rings sealing. Once the engine is bedded in, it is changed to AD oil for normal service.

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