The comments to your question are quite on point, and could probably serve as answers. I'll suggest that you also consider something else - which might not truly fix your issue, though.
If you find yourself repeatedly having your characters take part in "an informal conversation", why isn't it already clear to the reader, that that is what is happening?
This could be a case of 'show don't tell', as in, make it clear from the behaviour, topics, and language of the characters taking part, whether it is formal or informal.
Now, the reader may not actively experience the conversation, and in that case, the following doesn't help as much, however, if you're unsure how to make a conversation seem formal/informal, think of the difference between how you talk to a friend, a sibling etc, and perhaps a doctor, a teacher, your boss, whatever...
If that's difficult, I'll give a few examples of things that can make a conversation seem/feel informal;
- Joking, whether it's inside-jokes, freestyling, being silly...
- Allowing random interruptions, somewhat equally from the different parties involved. (In formal settings, usually there's a superior or semi-authority - interruptions would generally be tolerated from that person, not against them).
- Changing the topic, talking about private or non-work-related things (or not related to which-ever context the conversation takes place.)
- Flirting (though, it does happen in formal settings) - Can be closely connected to joking.
- Having no clear goal with the conversation, like "what's the plan for the sale of x, the strategy of x war, the implementation of the new computers in classroom 3" and so on...
- Questioning/challenging opinions, suggestions, understandings, and so on...
- It is generally more relaxed - people usually associate informal conversations with being themselves, rather than meeting certain expectations of 'appropriate behavior'.
These are just some of the things that often characterise informal conversations. That being said, all people converse differently, uniquely, and some people have a tendency of making otherwise formal settings slightly less formal. They implement some of the things above for different reasons. Maybe they can't help themselves but make that joke, maybe they purposely joke about the thing that everyone else takes (too) seriously, maybe they know that their boss is also just a human being, meaning that their understandings, experiences and input are of equal value, and so on...... This could/should be considered or naturally shown when your characters interact.
So, to sum up: maybe the issue isn't that 'informal conversation' should sometimes be swapped with 'chat, small-talk, catch-up, gossip'-what-have-you, but that something is being told, which would benefit from being shown.
Upon re-reading the title of the question, I realize that this answer may in fact be slightly off-topic - here's a counter-point; Even if your 'informal conversation' was simply an example, the same thing could apply to any other situation. In case my answer is of no use to your particular situation, hopefully someone else will find it useful.