On Thu, 9 Nov 2017 07:55:29 -0800 (PST)
Jonathan wrote:

My sister-in-law has a rocking chair that creaks annoyingly.

See picture for mechanism:

https://ibb.co/ncvMaw

So far as I can see the bolt has a spring around it that controls the
rocking mechanism. She has tried oiling it to no effect.

Would greasing the spring work?

Any thoughts welcome.

It doesn't look like it's the original bolt - if it's under-size it
might be moving in a hole where it should be snug.
There's probably a metal insert between the spring and the wood,
greasing that might help.