Thinner oil oftenly results in more vibrations, so nothing surprising there.
I feel the difference even between 10W-40 and 20W-40.
And have strong feeling that 20W-50 will give me even smoother ride. But that damn clutch of GS keeps me from going to 20W-50, it fails to do its job in the morning with too thick oil and need painfully long warm up before it comes back to its senses.
That being said, yes i used hiflo once but those days i was using kixx 4T as routine oil. Both are 20W-50 and both gave smooth ride.
But now i don't do experiments much and sticking to Motorcycle Oils only.
And after much khwari, found Zic M5 pretty much hit the sweet spot on my GS.
Though sometimes i think to try out Castrol Activ 20W-50.