It even says so on the lid to the brake - fluid reservoir - which the mechanic had also covered in black oil so I had to clean the lid to read the text. (mechanics.stackexchange.com)
High - end wild bird food with no fillers, especially those with black oil sunflower seeds, will bring in the most species. (petbusiness.com)
He applies ink and thin lines of black oil. (haberarts.com)