The meaning of WHEN

The meaning of WHEN

The sentence below is from Capital by Karl Marx.

Labour adds oil to the lamp of life, when thinking inflames it…

I want to ask the exact meaning of the underlined when.

I looked up The Free Dictionary for its various meaning.

1. At the time that: ? in the spring, when the snow melts.
2. As soon as: ? I'll call you when I get there.
3. Whenever: ? When the wind blows, all the doors rattle.
4. During the time at which; while: ? When I was young, I was sick all the time.
5. Whereas; although: ? She stopped short when she ought to have continued.
6. Considering that; if: ? How can he get good grades when he won't study?

Among these meanings I think #5 and #6 seems to be similar to the when in question. I thinks #5 is more suitable to the context, but I’m not sure enough not to post this question.

Thanks in advance.