On the Vintage and Antique section of the Board I've seen mention many times of synthetic TriFlow lubricant as being superior to sewing machine oil for lubrication. Other oils can harden into brown gunk over time. Old sewing machines were commonly wiped down on the outside with sewing machine oil to preserve them, which has created a dark brown gummy coating. I have many treadle machines waiting for a good cleaning. Sewing machine oil should dissolve the gunk on the outside without harming the beautiful gold decals, and I'll be using TriFlow inside.