There are two different types of capacitors. One is a motor run capacitor. The other is a motor start capacitor. Mostly, a motor run capacitor is needed for a furnace blower motor. Almost all motors will recommend a capacitor or one might be included with a motor purchase. In some events, a motor run capacitor may be out of stock. However, as long as a capacitor is within 5% of the recommended rating, then a substitute can be used.