1. List casters should take approximately 80% damage from a single pour of oil. Heal-able, but definitely painful.
2. Healer/hybrid types should take ~60%.
3. Full tanks should take ~40%.
This should be coupled with much higher hit points on the oil and it shouldn't actually be unusable at 20% as it is currently. Make the oil only melee/siege damageable and no longer able to be casted upon. Keeps with double oil would still be able to kill non tanks with a timed pour.
Just my 2copper after playing the siege game for a while.