M3's are just oil burners, Don't stress yourself out.
Try switching to motul 10w60, I did this and got positive results. My M3 car has a VAC low compression motor paired with a VF supercharger, it uses a quart every 700ish miles. This number has gone up with the motul.
Also in Bmw's TSB they say it's considered normal for the s65 to be consuming a quart per 600 miles.
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