Indoor fuel tank removed - How to eliminate lingering smell
I wouldn't do a "controlled burn". If the oil penetrated deep into the wall structure. You could create more problems than you have already.

If you want to plaster the walls and protect the new plaster from the oil on the walls, for belt and braces I would use an oil based paint and cover the affected areas in the paint. If you are not sure how much of the wall is affected, paint the whole wall.

Don't immediately plaster over the dry paint though as the paint here needs additional time to set.

Once the paint has had enough additional time to set, you can jump right in and start applying the priming PVA which will help the plaster adhere firmly to the wall.

It is also advised that you select the correct plaster undercoat for painted walls, for example, see Table 1a (Plaster  selection - PDF page 7) in https://www.british-gypsum.com/~/media/Files/British-Gypsum/White-Book/White-Book-C07-S02-Linings-Plaster-systems.pdf?la=en