É permitido. Quanto a se é uma boa ideia ...
Nas regras, há uma tabela Miscibilidade de poções que se aplica igualmente a óleos. Basicamente, se você misturar poções (ou óleos), você rola nessa mesa para ver o que acontece:
(01) EXPLOSION! Internal damage is 6-60 h.p., those within a 5" radius take 1-10 h.p. If mixed externally, all in a 10' radius take 4-24 hit points, no save.
(02-03) Lethal poison results, and imbiber is dead; if externally mixed, a poison gas cloud of 10' diameter results, and all within it must save versus poison or die.
(04-08) Mild poison which causes nausea and loss of 1 point each of strength and dexterity for 5-20 rounds, no saving throw possible.; one potion is cancelled, the other is at half-strength and duration.
(09-15) Immiscible. Both potions totally destroyed, as one cancelled the other.
(16-25) Immiscible. One potion cancelled, but the other remains normal (random selection).
(26-35) Immiscible result which causes both potions to be at half normal efficacy when consumed.
(36-90) Miscible. Potions work normally unless their effects are contradictory, e.g. diminution and growth, which will simply cancel each other.
(91-99) Compatible result which causes one potion (randomly determined) to have 150% normal efficacy. (You must determine if both effect and duration are permissible, or if only the duration should be extended.)
(00) DISCOVERY! The admixture of the two potions has caused a special formula which will cause one of the two potions only to function, but its effects will be permanent upon the imbiber. (Note that some harmful side effects could well result from this...)
Esse é o tipo de grandiosidade que a AD & D teve, que, infelizmente, IMO, foi eliminada por ser muito difícil para os jogadores. No entanto, também IMO, jogador estúpido o suficiente para fazer experimentos de química com produtos químicos mágicos obter o que eles merecem.