Some keeps provide a unreasonable difficulty to hit the oil on the outter wall.
Biggest offenders are Karak karak (TM) and Southern Garrison (Praag).
The jump-instacast trick works in these keeps only in a very limited area.
Those areas are so small that only 3-4 rdps can do their jumptrick there before collision blocks you from jumping.
Defenders have it incredible easy to pressure that area and grinding the damage on oil to a virtual standstill.
Souther garrsion example:
Jumping helps by giving you a better angle and not by getting you closer (see graphic example)
a) Implement some gameobjects that are climbable like dirtpiles or rocks or debris to give more area to target the oil from (could create issues with pounce like abilities)
b) Move the Oil forward so the coordinate that is used for the LOS check doesn't get obstructed by the keep edge.
c) Change the LOS coordinate in the Oil gameobject upward so you get a better angle.
d) Change the Gatehouse texture (unreasonable)
In my personal opinion this is a major problem since it makes certain keeps unsiegeable with any noteworthy defending garrison because of a technical issue.