A direct hit from flaming oil burns for two rounds, causing 2d6 points of damage in the first round and 1d6 points in the second round. Poison Poisons that are only effective when injected or ingested are generally not effective as grenade contents. It is very hard to get the poison to splash into a targets mouth and a direct hit can't guarantee this. The exception is a huge creature that has a nice gaping mouth as a target. A direct hit into the mouth would suffice to cause the poison to be considered injected.
Contact poison is the preferred type of poison because they have normal effects.
In fact, to fill a whole grenade with contact poisons is very expense considering a small dose could be as little as a drop and cost a small fortune. So, characters may waver the splash effects of the grenade with the use of less poison and hope for a direct hit which would be enough to administer the poison.