Cylinder oil base number has to match the sulphur in the fuel: too little alkalinity and liners corrode, too much and deposits build up. This guide explains the current engine maker guidance, how to set feed rates, and how drain oil analysis tells you whether the balance is right.
What BN does
When fuel burns in a two-stroke engine, its sulphur forms sulphur oxides, which combine with water to form sulphuric acid. Cylinder oil carries alkaline additives that neutralise this acid on the liner surface. The base number, or BN, measures that alkalinity. It is not a measure of the oil's cleaning ability or other properties, which depend on the rest of the additive package.
The aim is to supply just enough alkalinity for the acid produced.
Two ways to get it wrong
- Too little BN for the fuel sulphur: acid is not fully neutralised. Where liner surface temperatures fall below the acid dew point, acid condenses and attacks the liner and rings, a process usually called cold corrosion. The result is accelerated wear.
- Too much BN for the fuel sulphur: unused alkaline additives build up as hard deposits on the piston, including the ring lands. These can disturb the oil film and cause scuffing and wear of their own.
Both develop over hundreds of running hours and are easy to miss until an inspection or wear measurement reveals them, so BN selection, feed rate and monitoring must be managed together.
Matching BN to fuel: current maker guidance
Start from the engine maker's current service letter for the engine type. MAN Energy Solutions, now Everllence, issued updated cylinder lubrication guidance for its B&W two-stroke engines in June 2023 (SL2023-737). In summary:
- Fuels at or below 0.50% sulphur, including VLSFO, 0.10% fuels and MGO, and gas or methanol operation: a BN 40 oil. Newer engines are directed to a higher-performance category of BN 40 oil, while older engines may use either category, or continue on a BN 100 oil of the higher category where experience with it has been good.
- Fuels above 0.50% sulphur, meaning HSFO burnt with a scrubber: a BN 100 to 140 oil of the higher category, on older engines alternatively a BN 70 oil of the standard category, or a blend of low and high BN oils mixed on board.
- Oils below BN 40 are no longer recommended for these engines.
WinGD takes a similar approach: the higher the fuel sulphur, the higher the BN, within published ranges and using oils from its validated list. Whatever the engine, check that the oil ordered is on the maker's approved or validated list.
Ships that switch between fuels
A scrubber-fitted ship that burns HSFO at sea and compliant fuel in ports or ECAs has two routes. The Everllence letter allows a single high BN oil to be used on all fuels, including inside a sulphur ECA, where experience on that engine supports it. Otherwise the ship carries a high BN oil and a BN 40 oil, or a blending system, with a clear procedure for when to switch. Ships without scrubbers that alternate between VLSFO and MGO can stay on the same BN 40 oil, since both are at or below 0.50% sulphur.
For HSFO engines, Everllence recommends confirming the choice with scavenge port inspections before entering and after leaving a sulphur ECA during the first few round trips.
Optimising the feed rate
Cylinder oil is expensive, so there is pressure to cut the feed rate. Reductions should be gradual and backed by evidence.
- For low-sulphur operation, the Everllence letter sets an absolute minimum of 0.6 g/kWh, a normal minimum of 0.6 to 0.8 g/kWh (0.8 g/kWh with a controllable-pitch propeller), a safe setting of 1.2 g/kWh and a maximum of 1.5 g/kWh.
- WinGD gives a correct range of 0.6 to 1.2 g/kWh, with 0.9 g/kWh as a starting point for initial screening.
- Reduce in small steps, 0.05 to 0.1 g/kWh in the Everllence letter, and only once cylinder condition and drain oil iron levels confirm it is safe.
- For high-sulphur operation, the feed rate is linked to fuel sulphur and oil BN; follow the maker's HSFO guidance for the engine type.
Drain oil and scrape-down analysis
Scavenge drain oil, also called scrape-down oil, shows what is happening on the liner. The two key readings are residual BN, which shows how much alkalinity is left, and iron content, which indicates wear.
WinGD's guidance for oils of BN 40 and above illustrates how results are read:
- Safe: residual BN between 25 and 50, and iron below 200 mg/kg.
- Alert: residual BN between 10 and 25, which suggests a corrosion risk; residual BN above 50, which suggests a deposit risk; or iron between 200 and 500 mg/kg.
- Danger: residual BN below 10 or iron above 500 mg/kg, which requires immediate action on feed rate or BN.
Everllence's drain oil guidance, issued in November 2025 as SL2025-776, recommends accredited laboratory analysis every 1,500 running hours or three months, with onboard testing in between as an early warning. It also stresses that iron readings should be corrected to a reference feed rate and read together with fuel sulphur and residual BN, since dilution by system oil can lower both figures.
A practical routine
- Confirm the fuel sulphur for each tank from the BDN and laboratory results.
- Select the oil and BN according to the current service letter for the engine type.
- Set the feed rate within the maker's range and record it.
- Sample drain oil regularly, onboard and through an accredited laboratory, and trend the results.
- Inspect through the scavenge ports at the intervals the maker recommends, and after any change of fuel or oil.
- Adjust feed rate or BN only in small steps, and record the reason for each change.
Planning lube supplies alongside fuel stems helps ensure the right oils are on board before a change of trade. Atlas Bunkering handles lube oil supplies for owners and managers on the same owner-side basis as fuel, proposing only trusted suppliers and passing on the rate secured.
Questions and answers
Can I use BN 100 oil when burning VLSFO?
Only if the engine maker allows it for that engine type. Current Everllence guidance recommends BN 40 for fuels at or below 0.50% sulphur, with BN 100 of the higher category accepted on older engines, and a single high BN oil on scrubber engines, where experience has been good. Monitor closely for deposits.
Do I need a different cylinder oil for MGO in an ECA?
Usually not, if the ship already runs a BN 40 oil on VLSFO, since both fuels are at or below 0.50% sulphur. Scrubber ships on a high BN oil either switch to BN 40 or, where the engine maker allows it and inspections confirm good cylinder condition, keep the same oil.
How often should drain oil be analysed?
Everllence recommends accredited laboratory analysis every 1,500 running hours or three months, supplemented by onboard tests. Follow the specific guidance for your engine.
Sources
- MAN Energy Solutions Service Letter SL2023-737: Cylinder lubrication update
- WinGD: Lubricants, fuel, lubricants and water guidelines
- Marine Log: New Everllence guidance on two-stroke engine drain oil monitoring
This guide is general information, not legal or technical advice. Check the current rules and your engine maker's guidance for your vessel.