Can't remove grinding stone on Komo classic

Good morning,

I purchased a used Komo classic mill. The person who had it previously said it hadn't been used in a few years. It works, but I tried to remove the bottom stone and was unable to. I think screw is stuck. Even my brother was unable to unscrew it. He suggested maybe putting a drop of oil where the screw is, but I was afraid to do that because I don't want to mess up the stone. Does anyone have any suggestions?

I would like to be able to clean the housing and I also am interested in purchasing the interchangeable stones.

Thanks,
Elaine