What is the minimum top up for an Oyster card?

Is there a minimum top-up for an Oyster card? Seriously, that £20 auto top-up drives me nuts! It's so annoying! Like, suddenly, bam, £20 gone. Okay, I know it's handy, and it saves you from that panicked scramble at the barrier when your balance is low. But still! I often only need, like, a fiver at a time for a quick trip or two. Why can't TfL just let us choose a lower auto top-up amount? What's the deal? Ten pounds? Even that would be a massive improvement. Honestly, it feels like they're pushing you to keep a load of money on your Oyster, which, well, I'm not always comfortable with. Remember that time I lost my Oyster card? Nightmare! Had like, almost a full £20 on it. Gone. Poof. If I'd only topped up a tenner... you know? It wouldn't have been such a big deal.