A replaceable fluid container for an engine or a vehicle, the fluid container being configured to cooperate with a dock, the replaceable fluid container comprising: a fluid reservoir; at least one fluid port adapted to couple with a fluid circulation system; and an actuator, configured to be operated between a first condition and a second condition, wherein the actuator is configured, in the first condition, to enable the fluid container to be inserted into and/or held in the dock, in a seated but undocked condition, and inhibit docking of the fluid container to the dock, and wherein the actuator is further configured, when operated from the first condition to the second condition with the fluid container being in the seated but undocked condition, to enable the fluid container to dock in an engaged condition with the dock, and wherein the at least one fluid port comprises a coupling adapted to connect to a corresponding coupling (7, 8) of a fluid circulation system associated with the engine, such that when the couplings (7, 8) are being connected, a seal is made between the connecting ports before any valves open to allow fluid to flow, and wherein the actuator comprises a fastening mechanism configured to cooperate with a fastening mechanism of the dock, the actuator being configured such that: in the first condition, the fastening mechanism of the actuator is configured to abut with the fastening mechanism of the dock to hold the fluid container such that the fluid container is in the seated but undocked condition, and in the second condition, the fastening mechanism of the actuator is fastened to the fastening mechanism of the dock and the fluid container is docked in the engaged condition with the dock, wherein the fluid is an engine lubricating oil. a fluid reservoir; at least one fluid port adapted to couple with a fluid circulation system; and an actuator, configured to be operated between a first condition and a second condition, wherein the actuator is configured, in the first condition, to enable the fluid container to be inserted into and/or held in the dock, in a seated but undocked condition, and inhibit docking of the fluid container to the dock, and enable the fluid container to be inserted into and/or held in the dock, in a seated but undocked condition, and inhibit docking of the fluid container to the dock, and wherein the actuator is further configured, when operated from the first condition to the second condition with the fluid container being in the seated but undocked condition, to enable the fluid container to dock in an engaged condition with the dock, and wherein the at least one fluid port comprises a coupling adapted to connect to a corresponding coupling (7, 8) of a fluid circulation system associated with the engine, such that when the couplings (7, 8) are being connected, a seal is made between the connecting ports before any valves open to allow fluid to flow, and wherein the actuator comprises a fastening mechanism configured to cooperate with a fastening mechanism of the dock, the actuator being configured such that: in the first condition, the fastening mechanism of the actuator is configured to abut with the fastening mechanism of the dock to hold the fluid container such that the fluid container is in the seated but undocked condition, and in the second condition, the fastening mechanism of the actuator is fastened to the fastening mechanism of the dock and the fluid container is docked in the engaged condition with the dock, in the first condition, the fastening mechanism of the actuator is configured to abut with the fastening mechanism of the dock to hold the fluid container such that the fluid container is in the seated but undocked condition, and in the second condition, the fastening mechanism of the actuator is fastened to the fastening mechanism of the dock and the fluid container is docked in the engaged condition with the dock, wherein the fluid is an engine lubricating oil.