Murky Oil of Speed doesn't grant the imbiber the beneficial effects of Oil of Speed, it confers the status effect of feeblemind, which lasts for 5 turns.
This item appears in all games, be careful it shows up exactly the same name as Oil of Speed in game. Combining it with a stack of Oil of Speed will turn the whole stack into Murky Oil of Speed.
|“||Potions are typically found in ceramic, crystal, glass, or metal flasks or vials. Flasks or other containers generally contain enough fluid to provide one person with one complete dose to achieve the effects of the potion.||”|
|“||This oddly murky oil would appear to 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 1 hour.||”|