I was recently flying on a A320-251N and directly after landing & coming to a halt at the gate, a maintenance person approached the engines carrying a box of something that looked like paint cans, opened some lid on the side and apparently put some of the content of the cans into the engine or so. What was it probably? (It was on an cold & icy airport, if that plays a role)
It was probably Engine Oil.
On modern jet engines, oil is a consumable in the sense that it gets burned up and needs topping up. A new engine will require less, a high time engine more. The JT-9s on older 747 were notorious oil-guzzlers. CF-6s were a bit better but they still needed topping up after an 8 hour flight.
Jet engines never get an oil change. The regular topping-up of burnt oil keeps the oil fresh and regular filter changes ensures material from engine wear is not circulated. Oil samples are taken at set intervals and sent for spectrographic analysis. Too much of a particular element will focus attention on a particular component. There are also 'Magnetic Chip Detectors' which trap ferrous metals and this also helps to pinpoint wear in the engine.