I think some of it is just sewing machine oil film itself. I am a nonsmoker. I bought a Kenmore brand new in 1980. It sat in the case most of the time, and in my back bedroom on a desk the rest of the time, not near any smoke, or cooking oils. It still turned amber-ish after about 20 years. I scrubbed it off with Tuff Stuff, and waxed it. Looks like new.