Chris Fulstow says he uses 'that' for restrictive clauses and 'which' for non-restrictive. Having no idea about the difference between restrictive and non-restrictive clauses, I looked this up, and it looks like Chris is right on the money.
Example (from
here)
Correct Restrictive Use:
Quote:
The store honored the complaints that were less than 60 days old.
Correct Nonrestrictive Use:
Quote:
The store honored the complaints, which were less than 60 days old.
These sentences have different meanings as well as different punctuation. In the restrictive sentence, the store honored only those complaints less than 60 days old, but not those over 60 days old. In the nonrestrictive sentence, the store honored all the complaints, all of which were less than 60 days old.
As you can see, there's also a difference in punctuation.
Re my original example, I guess this means "that" is correct. i.e. "It's a job that pays well." If we used "which" (and a comma), it would have changed the meaning: "It's a job, which pays well." This says, basically, "yes, it's a job, and jobs pay well."