Castrol Marine has welcomed new recommendations to ship-owners from MAN Diesel & Turbo, the world’s largest supplier of two stroke marine engines, supporting Castrol’s longstanding advice that misapplication of mid-range Base Number(BN ) cylinder oils can cause corrosion.

The engine manufacturer recently issued service letters that recommend “cylinder lube oil with 70BN or higher” for latest generation super long stroke engines using higher sulphur fuel (above 1.5%-2%).

 MAN said it “cannot recommend” cylinder oils “with a BN level between 50 and 60” for these engines .Ship-owners have been persuaded to try a host of new mid-range lubricants on the basis that they offer a solution across the range of sulphur content in fuel oil, including during slow steaming operations.

In contrast, Castrol Marine introduced Cyltech 80AW earlier this year, an 80BN oil addressing lubricant performance issues identified under slow steaming conditions when using HFO towards the top end of the permissible range. In a letter to customers on lubricant performance across its engine portfolio, MAN explained of 50-60BN oils: “oil wear varies significantly between engine types and between operational conditions”. Although mid-range lubricants were sometimes reliable “the test results from the mid-range cylinder oils can therefore not be transferred to general guidelines."

Another letter from MAN questions the ‘mid-range fits all’ proposition.It noted :“At present, we cannot recommend these new type oils on our newest engine designs: Mark 9 and further e.g. S80ME-C9; all G-engines.”

According to MAN: “MAN B&W two-stroke design is therefore based on optimisation of the amount of a BN70 cylinder oil for fuel sulphur content above1.5-2% and a BN 40 cylinder oil for lower sulphur content.”

Oils with a viscosity lower than SAE 50 are also not recommended for Mark 9 or G-type engines. For more than a year, Castrol Marine has advised owners that the use of mid range BN cylinder oils when slow steaming could lead to cold corrosion.

“We welcome these recommendations from MAN,” said Paul Harrold,  Castrol Technology Manager, Marine & Energy Lubricants.

“They support the exhaustive bench tests and shipboard trials that led to our own recommendations on cylinder oil selection in the slow steaming era. The recommendations we make to customers must always be based on sound science.

”Some cylinder lube oil suppliers have suggested that a single mid-range BN cylinder oil is sufficient for use with all marine fuels, under all operating circumstances. Castrol believes that customers faced with the prospect of only one grade of cylinder roil to cover all operating scenarios may be putting themselves at a disadvantage, especially if they regularly use fuel at the upper and lower ends of the permitted sulphur spectrum.”

MAN stated: “The design philosophy of two distinct lube oils is based on observations and tests showing that over-lubrication leads to over-additivation with consequential risk of calcium deposit formation and total suppression of corrosion, decreasing the scuffing tolerance. Moreover, under-lubrication may lead to under-additivation, which will lead to corrosive wear in cylinder liners and piston rings.”

Beyond 2015, MAN recommends BN 70-80 (-100 cylinder oils for fuels with up to3.5% sulphur content and low BN oil in SOx-ECAs between 2015-2020.

By 2020-25, the engine major envisages fuels being used in combination with scrubber technology.

“In case of scrubber solutions, high BN cylinder oil is to be used - ideally a BN 80-100.”In cases where “very low sulphur content in fuel” is anticipated, “BN 10-20 cylinder oils are needed."

“Our technical arguments have been challenged by a series of competitor productlaunches of lubricants conceived before slow steaming was even adopted,” said Mr Harrold. “This direct advice from MAN corroborates our own tests on the real issue."