An exemplary fluid heating method includes, among other things, operating an engine of a vehicle to heat an engine fluid during a drive cycle if the vehicle is in a fluid maintenance mode, and heating the fluid prior to the drive cycle if the vehicle is in the fluid maintenance mode and the vehicle is electrically coupled to a grid power source. An exemplary fluid heating assembly includes, among other things, an engine of an electrified vehicle, and a fluid heater that is activated to heat a fluid of the engine in response to the electrified vehicle being in a fluid maintenance mode when electrically coupled to a grid power source.