I had asked the same a while back, and got mixed replies... but, here's what I've noticed so far:

I thought I would see a slight drop in oil pressure when I switched to the 10-30. I didn't.

I have used Castrol GTX 10W-40 in my rig for over ten years, and my last change I went with the GTX 10W-30 . Used to be when I changed out the 40, the oil was NASTY looking dark brown and smelt bad. I guess I got used to it that way.

Now, judging by what I see on the dipstick, after 500 miles on fresh 10W-30, that oil is still pretty "golden", NOTHING like what the 40 looked like after the same amount of time.

I'm stickin' with the 30 .