Above: System oils are also sometimes used to lubricate stem shaft bearings.
Left: Cross head engines use system oil in the crankcase.
Info Centre >> Fuel and Lube Oil Technical Manual >> 7. Lube Oil - Test Results >> 7.2 Diesel Engine Oil >> 7.2.6 System Oil
This type of oil is used in the crankcase of large two stroke cross head type engines where the oil is protected from contact with combustion gases. The piston rod is constrained to move vertically with all angular motion being taken by the cross head. It is thus possible to seal all combustion products away from the crankcase.
System oils were traditionally straight mineral oils with some rust and oxidation protection. Low overbasing at around 1.5 TBN was seen to provide a reserve against strong acid reaction from scavenge drainage entering the crankcase via the stuffing boxes on the piston rod. Modern system oils typically have a higher TBN and a value of around 6-8 TBN is common. Not only do they lubricate all crankcase bearings, but also gear or chain drives and they often act as the heat transfer media for piston cooling.
Leakage of cylinder oil drains can cause the TBN of system oil to rise and this may also be accompanied by an increase in viscosity and insolubles. System oils may also be used as the lubricant for stem shaft and other bearings in the propeller shafts of large ships. Water is the only serious contaminant in this application.