Every forced air furnace has a blower motor attached to a fan that circulates the air; however, there are two types of drive systems. One is the newer model of direct drive, where the motor is attached directly to the fan blower. The other one is the traditional and older style belt drive. Many, but not all, direct drive motors have sealed bearings and require no lubrication, but virtually all belt drive fan motors must be lubricated annually.
Locate the position of your blower motor on your furnace. It may be behind a gravity panel that you can simply lift off, or there may be an access panel that you must unscrew with either a Phillips or flathead screwdriver. Remove the screws with a counterclockwise rotation.
Inspect the blower motor for the oil ports, which are located directly on the side and are sticking upwards. They may have red or other colored plastic caps that cover them. An oil port is on both sides of the motor.
Remove the plastic caps, if needed, and drip a few drops of bearing oil down into the oil ports, which is very subjective because there is no hard or fast rule unless you have a manual that specifically states how many drops of oil go into each port. Don't overfill the oil ports, which leads to leakage around the motor bearings and leaves an oil stain on the floor.
Replace the plastic caps over the oil ports as well as any access panel that you have removed, and do this again at the same time next year.