I can only say that delo 400 le makes my diesel very quiet. I can't prove it's the heavy dose of boron but something sets it apart from other HDEO's I've tried.
I'm currently trying kendall super-d which has about 100 ppm boron and 100 ppm of titanium. It sounds the same as delvac, super tech and rotella.
When using delo 400 the quiet/smooth sound is stunning. The biggest difference is while it's below operating temp, but even at full temp there is a noticable difference.
It stays quiet throughout the entire oci despite the boron levels dropping to about 90-120 ppm by the 20,000 mile mark. That's all I know...but like I said, I can't prove it's the boron.