When this oil is applied to skin, cloth, or any other material, it confers virtual invulnerability to acid. 

The oil will not wear off quickly; an application lasts for one full day before becoming impotent. 

However, each time material or flesh is exposed to acid, the potency of the oil is diminished by as many minutes as the acid would have caused points of damage to exposed flesh. Thus, if a black dragon of largest size and greatest age breathed upon a person protected by this oil, each breath would lower the oil's remaining protection time by 64 minutes, or 32 minutes if a successful saving throw versus breath weapon is made. 

Each flask contains sufficient oil for one man-sized creature (and accoutrements) for 24 hours. 

Or, 24 such man-sized creatures could each be coated for one hour's time; any combination of number of creatures and duration of potency between these extremes is also possible. (A horse is equivalent to eight man-sized creatures.)