Grease consists of oil and a thickener. The oil isn't thickened, it is just suspended as in a sponge. Grease is used whereever it is too difficult or impossible to use an oil circuit for lubrication. In addition grease must seal against contamination, remain stable during flexing, and be conveyable. The consistancy ranges from fluid to solid, classified as 000,00,0,1,2, and 3.