I was careful to start my post with "in American usage". Yes, we still use "conventional units" in engineering in the United States, even though the country is legally metric. "Imperial units" is a British term.
However, if a European or British engineer were working in my lab (before I retired), I would not expect him to use "mils" in the American sense, but would show him what sizes of wire are commercially available here.
I still oppose the use of "mil" or "mm" to mean "mm2".
It can take a long time to learn all the slang terms used in a foreign country.
By the way, on that multi-week business trip to UK, I ran up a 2240 GBP hotel bill (including meals), which I reported back to my company in the US as a "ton of money", which is approximately a "tonne of money". In the US, we call that a "long ton", while 2000 lb av is a "short ton" or just "ton". I still don't understand the British definition of a cwt = hundredweight as 8 stones, which is less than 100 lb.