I think it would be corrosion and oxidation inhibitors. These are nice to have in an oil, but not required. They are normally less expensive than other additives or base oil. Just a WAG.
I can't say that I've ever seen it used in Lubrication literature. Can you provide an example?
Otherwise I will have to agree with Dan...dyes,fragrances, waxes from insufficient refining...
A lube oil and especially greases may contain fillers, usually an inorganic element such as zinc oxide, copper oxide, molybendeum disulfide, graphite flakes. Some organic fillers are teflon.
Teflon is usually considered a thickening agent in some greases, but can also be considered a filler. When used in oils, it's only use is that of a sound deadener.
Zinc oxide or copper oxide may be used in greases for pipe fitting applications when threads are cut, and for drill rod applications. Another term for these types of fillers is "anti-sieze" compounds.