If the fan motor still has the black plastic dust cover covering the the top bushing it can be a challenge to get oil in there. The cover does unsnap from the motor, but they can be fragile after years of heat. Here's a picture of that dust cover to give you an idea. Like Simeon said, there's definitely some past threads on this topic and interesting solutions other folks came up with. If you pull the box, you can unclip the fan motor from it's cage and clean it really well and lube it up easily. I had one motor that I thought was completely seized up. A little TLC and it's been working fine for a long time. Good luck