Marine lubricants

The choice follows the fuel, not the engine

The main job of alkalinity (BN) in a cylinder oil is neutralising the sulphuric acid formed from fuel sulphur. Since the IMO 2020 limit, most of the fleet burns 0.5 % sulphur fuel, and a high BN has stopped meaning "more protection": the unused calcium stays in the system and forms deposits, with bore polishing as the typical result. Scrubber-fitted vessels stayed on high-sulphur fuel and need exactly the opposite. The second distinction is architectural: low-speed two-stroke crosshead engines have entirely separate circuits — cylinder oil is consumed in total-loss lubrication, system oil circulates and lubricates the bearings — while medium-speed four-strokes use a single trunk-piston oil that does both jobs.

Grades, BN and environmental requirements

For low-speed two-strokes, BN 40 cylinder oils cover operation on compliant fuel, while BN 70–100 remain for scrubber-fitted vessels burning high-sulphur fuel; system oils are typically BN 5–8 with high demulsibility requirements, since seawater inevitably reaches the sump. Medium-speed four-strokes run TPEO, usually BN 30–40 depending on fuel, and auxiliaries burning distillate need a lower BN than the main engine — a difference often overlooked. On the regulatory side, systems with an oil-to-sea interface, the stern tube above all, require environmentally acceptable lubricants (EALs) in US waters under the Vessel General Permit. Deck equipment and cranes come back to HVLP hydraulics and water-resistant EP greases.

Expensive mistakes on board

Most of them come from treating BN as a measure of quality:

A high-BN cylinder oil kept on after switching to 0.5 % sulphur fuel — the unused calcium forms deposits and leads to bore polishing.
System oil used as cylinder oil, or the reverse, on a crosshead engine where the two circuits are separate precisely because the products differ.
A TPEO chosen on BN without checking what the auxiliary engine actually burns.
Entering regulated waters without an EAL in the stern tube — a non-conformity with administrative rather than technical consequences.
Cylinder oil feed rate left at the default instead of being tuned from ring inspection and scrape-down analysis.