What BN cylinder oil should I use with VLSFO or LSFO?

Low-sulphur fuels generally call for a low-BN cylinder oil of the order of BN 40. Confirm the exact grade against your engine OEM's approval and your lubricant supplier's recommendation for your specific fuel sulphur.

What BN cylinder oil should I use with HSFO?

High-sulphur fuel generally calls for a high-BN oil — commonly BN 70, and up to BN 100 at the highest sulphur levels and feed rates — to neutralise the additional sulphuric acid.

What happens if the BN is wrong for the fuel?

Too low: sulphuric-acid (cold) corrosion attacks the liner. Too high for a low-sulphur fuel: unreacted additive forms hard deposits and can cause bore polishing. Both shorten component life.

Marine Cylinder Oil BN Selection

The base number (BN) of a two-stroke cylinder oil must be matched to the sulphur content of the fuel you are burning. As a rule of thumb used across the industry, high-sulphur fuel oil (HSFO, around 3.5% sulphur and above) calls for a high-BN cylinder oil — typically BN 70, and up to BN 100 for the highest sulphur and feed rates — because the oil's alkaline reserve must neutralise the sulphuric acid formed during combustion. Very low and low sulphur fuels (VLSFO and LSFO at or below roughly 0.50–1.5% sulphur, the post-IMO-2020 norm) generally call for a lower-BN oil of the order of BN 40. Marine cylinder oils span roughly BN 25 to 140 to cover this range. The danger runs both ways: too little BN for the fuel lets acid attack the liner (cold corrosion), while too much BN for a low-sulphur fuel leaves unreacted additive that forms hard deposits and can cause bore polishing. Operators manage cylinder condition by adjusting BN, feed rate, or both — and the engine OEM's approval and your lubricant supplier's recommendation always take precedence over any general guidance, including this page.

Match BN to fuel sulphur

Higher fuel sulphur → higher BN. HSFO commonly pairs with BN 70 (BN 100 at the top end); VLSFO/LSFO commonly pairs with BN 40. Engines run only on low-sulphur fuel may use a BN 40 grade exclusively. The exact grade depends on the specific fuel sulphur, engine model and the OEM's current category approval — confirm it, don't assume.

Cold corrosion vs over-lubrication

Under-based oil (BN too low for the sulphur) allows sulphuric-acid corrosion of the cylinder liner, seen as scuffing and accelerated wear. Over-based oil (BN too high for a low-sulphur fuel) leaves unreacted calcium that bakes into hard deposits, risking bore polishing and ring problems. Drain-oil analysis and regular scrape-down BN checks reveal which way the balance has tipped.

BN, feed rate, and switching fuels

Because many ships switch between fuels (e.g. entering an ECA), cylinder condition is tuned with both BN choice and feed rate. Feed rate is set within the OEM's minimum and is raised or lowered with load, fuel sulphur and observed liner condition. Follow the engine maker's service letters for the specific feed-rate window and any minimums for your engine mark.

Frequently asked questions

What BN cylinder oil should I use with VLSFO or LSFO?: Low-sulphur fuels generally call for a low-BN cylinder oil of the order of BN 40. Confirm the exact grade against your engine OEM's approval and your lubricant supplier's recommendation for your specific fuel sulphur.
What BN cylinder oil should I use with HSFO?: High-sulphur fuel generally calls for a high-BN oil — commonly BN 70, and up to BN 100 at the highest sulphur levels and feed rates — to neutralise the additional sulphuric acid.
What happens if the BN is wrong for the fuel?: Too low: sulphuric-acid (cold) corrosion attacks the liner. Too high for a low-sulphur fuel: unreacted additive forms hard deposits and can cause bore polishing. Both shorten component life.