Good advise from Makeithappen77.
Lionel whistle motors have two bearings.
Your whistle motor probably is one of the first two listed above.
The WS-85 has a bearing staked onto the whistle chamber cover plate.
If you have this style plate, I suggest placing the tender chassis on its side so the bearing is facing up, and placing one drop of oil on the ball bearing in the center. Then let it soak in for a while. If the oil all soaks in, add 1 more drop, and let it soak in.
Since your whistle was working well, this should be plenty.
Wipe away the excess with a rag or paper towel.
The other bearing is in the brushplate. There really isn't any bearing at all, just a hole through the fiber board.
Lionel instructed customers to use oil, but I like to use Labelle 106 grease. Unfortunately, this requires the wires to be unsoldered from the brushplate so it can be removed. I use a toothpick to put a light coating on the inside of the hole, and a light coating on the armature shaft.
I usually clean the brushes and commutator plate while the motor is apart.
I use odorless mineral spirits on a Q-tip, and a fiberglass eraser (if necessary) on the commutator plate.
The brushes and commutator plate should never have any lubricant on them.