well viscosity since its the main parameter needed for lubrication really.
actually my project aims at coming up with an alert system that warns you when your engine oil is used up or degraded(for lack of better words
) and takes a precautionary action to prevent stalling of your engine.
I know the mileage is the one usually used for the next oil change but its usually a bargain and does not factor in variable aspects.
for the temperature, it will always affect viscosity but there is always a threshold which when exceeded, indicates the oil needs to be changed.