The words in (1) are used in many different ways, some of which are synonymous and others of which are not. 'I'm feeling peculiar' means that I am feeling unusual-- usually a little ill; otherwise, (2) is unnatural. In (3) only the last two are good, and they are synonymous. As for (4), I use it, often about some posts. And the meaning in the first two sentences of (5) is also common but formal; the third example is possible, only used to mean the unusual nature of the opinion.
Hope this helps.