Traditionally vegetable oil was used for Japanese swords, usually camellia oil (tsubaki-abura 椿油) with a few drops (less than 5%) of clove oil (chōji-abura 丁子油). It has a yellowish color, and strongly smells of cloves. It absorbs moisture, but gums up easily when dried. It's also mildly aggressive (which is good for cleaning Japanese swords in a less-than-perfect state of preservation), but quite messy when it soaks into the sheath.
Mineral oil (kōbutsuyu 鉱物油) was used from the Meiji era on when mass-production of swords for the Japanese army started - it's cheaper and easily obtainable. It's clear, and thinner than vegetable oil, and doesn't deteriorate as fast; one also needs less of it compared to chōji scented vegetable oil.
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