I usually delay changing the oil until it's at or just below the minimum mark, and even then nearly four litres of the stuff still pours out of our Focus.

Well of course you can do whatever you like, as can I.

Cars are designed to be as foolproof as possible - they hold far more oil than they need. All experienced mechanics will have seen cars put in for service with the oil level barely on the end of the dipstick, let alone at the minimum mark, with no obvious undue engine wear.

"No obvious undue engine wear" eh?

That sounds like the old "no issues/nothing blew up" Gold Standard.

Long time ago my landlady asked me whether this light that had been on for a long time on the dash of her Fiesta was important.

Just a bit of bearing metal slurry on the end of the dipstick, but once I'd used MY oil to fill it up there was "no obvious undue engine wear."

Cars are designed to be as foolproof as possiible unless they aren't. Word on the forums is/was that the minimum level on the Charade engine is pretty marginal.

I don't know this for a fact, but I don't particularly want to confirm it the hard way.

Even if it isn't true, an oil leak could quite quickly put any engine in danger, more quickly if its already running on the minimum mark Running at the full line gives a safety margin.

Your faster oil warm up rationale seems valid and likely to be a real benefit in the UK in winter. Whether it justifies the slight risks involved is a judgement call.

Its much less likely to be a significant benefit here in Taiwan.