This smooth liquid appears to be oil of any sort — even possibly of poisonous nature. 

When applied to any matter which was formerly alive (leather, leaves, paper, wood, dead flesh, etc.), it enables that substance to resist the passage of time, each year of actual time affecting the object as if only a day had passed.

The substance never wears off, though it can be magically removed.

The object coated with the oil also gains a +1 bonus on any saving throws which must be made for it. 

There is sufficient oil within one flask to coat one horse, eight humans, or an equivalent area/volume of some other eligible object or substance.