I understand that some folks have shown some rather ugly examples. One particular example looked horrible on M1 0W-40. I'm calling that one out as NOT TRUE. I've seen thousands of engines, not a single regular M1 user had similar varnish. Not one. In fact, regular M1 users never have sludge, varnish, clogged piston drain holes or coked turbochargers.
What I have seen is people using 0W oils and changing at 12,000+ mile intervals and experiencing cam lobe, roller follower and timing chain wear. This should come as no surprise.