I usually clean the cases and cabinets with old fashion soap and water (well wringed cloth, nothing gets soaked), if there has been traces of mold I have wiped off with a cloth dampened in vinegar water too. Mildew should not reappear on the machine after general cleaning unless there is some degree of humidity. Is it stored in a heated room? I don't think a film of mineral oil (basic sewing machine oil) should cause mildew? After cleaning you can go over the machine with a dry microfiber cloth, should reduce oil residue.