It depends one the engine's internal clearance/tolerances and the type of oil.
Synthetic is naturally a bit thinner, so it's going to flow more freely and get through smaller clearances. Older engines are worn in and the seals aren't as good, so a thicker oil may be necessary. A brand new engine made by today's manfacturing tolerances can take the newer synth oil, and use a thinner version of it to boot! The C5's now ship with Mobil1 fully synth, 5w-30. I use that same oil in my ZZ4. No need to run thicker oil.
As always, there's plenty of conversations that branch off from this, such as heavier oils rob horsepower, but carry away heat better, blah blah blah. There are many educated opinions out there, and just as many claiming it to be fact. You just gather all the info you can and try to make your own educated decisions.