Abstract: A pressure differential indicator for indicating a change in a fluid pressure differential across a component in a fluid system, such as an oil filter, has piston means, means for communicating fluid pressures of said system upstream and downstream of said component to opposite sides of said piston means, respectively, and differential means associated with the piston means for normally balancing the resultant forces of said pressure on opposite sides of said piston means at a given predetermined pressure differential across said component whereby said piston means is responsive to an increase or decrease in the pressure differential above or below said predetermined differential pressure to indicate a clean or clogged filter condition. Means are provided for holding the piston means in its last position upon relief of said first and second pressures to indicate the operating filter condition when the system is not operating.