I never had problems with sludging and yet have a motor with #3 and #4 bearing spun. My motor looks like brand freaking new inside. Even after the motor started knocking, I did an oil pressure test and found 20psi at idle and 50 at 3000rpm and that's with the rod just pounding.

 

another reason to go full synthetic is, if yuu live where it gets cold, your engine will turn over like it's a warm day in july when it's -10 degrees. you'll put much much less of a strain on the starter and electrical system.

if you want to learn about oil, i highly recommend bobistheoilguy website, and his tutorial. then take the test at the end.