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I know a guy who used oil approved by BMW in his BMW E39 M5. The engine failed, and he did not get it covered by warranty. For that engine, only one specific Castrol oil (TWS 10W-60) was allowed, and he had used another type. You should check if there are any such restrictions for the M135i.
I think the main reason for approved oils is the long service intervals. When you replace the oil every 5000 km, pretty much any oil should work, as it will not have time to break down before it is replaced.
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