[Paull]

Typically, slow speed diesels do not oil have changes. The reason here is that they consume a certain amount of oil every passage (from port to port). At the beginning of each journey, the engine sump will be topped up, normally by about 500 litres. These constant additions keep the oil up to spec. And to change out the whole charge of about 25000 (ish) litres is far too expensive.

You should also note that the main lube oil system is seperate in that the cylinder and the crank case are seperated by a stuffing box. That is, the main lube oil does not get contaminated by the fuel. The cylinder liners are lubricated with a specificcaly designed lube oil that is injected into the liner via 'quills' and this oil is always being consumed, unlike the main lube oil system.

And yes, all slow speed diesels are 2 stroke. They have inlet ports in the cylinder liner and the exhaust is removed via a single exhaust valve