Oil of Sharpness
Source: Dungeon Master's Guide
Potion, very rare
This clear, gelatinous oil sparkles with tiny, ultrathin silver shards. The oil can coat one slashing or piercing weapon or up to 5 pieces of slashing or piercing ammunition. Applying the oil takes 1 minute. For 1 hour, the coated item is magical and has a +3 bonus to attack and damage rolls.