I’ve been giving a few thoughts to this noise issue too. I have a 1987 Jetta 1.8 (hydraulic valves) and have been using 5w-30 synthetic blend since 1988.
There is a concrete 5ft high divider that divides the opposing lanes on a road near my house. I used to remember driving with my drivers window open, one could hear the sound of the engine reflecting off the concrete (it’s right next to the left lane). And I used to think my engine sure does sound kind of noisy. Almost like an engine that had 250k miles one it, was very worn, and was burning a liter of oil every 500km. Back then that 5w-30 probably was a Grp I/PAO blend, and later in the 1990’s it was likely switched over to a Grp II/PAO and then a Grp II/III blend ~ 2000.
I drove my car yesterday and the engine certainly sounds quieter than it used too. Never gave it much thought these days until I read some of these threads about oil and engine noise. Although, I’m nearing the end of my current OCI and maybe my current 5w-30 has thickened up somewhat. Or maybe I just have to wait for a hot day to test my current fill on.
Is this a thinner oil issue, or a PAO issue? One thing that comes to mind from reading a little on various lubricants is the low coefficient of traction of PAO’s, which from what I’ve read is the lowest of any lubricant. Maybe it’s so slippery the hydraulic valves are slapping out quicker or something.
So, there does seem to be something to this noise issue. I was never really concerned about the noise, but just thought it was a little odd.
My 0.02