The lightning would be effective in "blasting" off small, concentrated patches. However, a chain of lightning could be effective in clearing a larger section. The max area would be 10ft by 5ft, however if she chains it with someone else it can be larger. Just a note that individually, she can only uncover that much. It is also the wind, if mixed with the lightning, could help keep the oil at bay for a little longer, up to forty-five seconds, before it all collapses and covers the wall again.

As for what Aerith may be able to see and hear from the scholars, it's clearly a mural hidden beneath the oil. It's very damaged and scratched up, but there are some depictions of land, cities, people, and more large, strange squiggly lines. It's all very vague, so the scholars will have to try and patch it all together and release their finding later.