Murky Oil of Speed doesn't increase the speed of the imbiber. Instead, it confers the status effect of feeblemind. The effects lasts for 5 minutes. This item appears in Baldur's Gate, Baldur's Gate: Enhanced Edition and Baldur's Gate II.
|“||This oddly murky oil would appear increase the movement and combat capabilities of the imbiber by 100%. Thus, a movement rate of 9, becomes 18, and a character normally able to attack once in a round attacks twice. This does not reduce spellcasting time however. The duration of the effect is 5 turns.||”|