Wasnt there a rumour that it is excellent in a new engine not so good in an old engine. Was the theory it is far more effective at preventing deposits but also removes these in an older engine so all of a sudden what had beded in well no long fits so good.
I have no idea whether these rumours are accurate.
I do know in my race engine when I rebuilt the engine before using sysnthetic it was pretty dirty after a few thousand miles, but next time I opened it up after running on synthetic with a change every 1,000 miles it looked new.