I know this is probably an argument from my generation, but just in case this train of though is floating around somewhere, your goal should be to use the lightest possible oil, not the heaviest. When I was getting into this stuff there were flying assumptions that if you had a powerful motor you had to use 20w50, and if it said 5w30 on your oil cap this was just stupid because a v8 needs 10w40 at least.
It all comes down to your tolerances as said above. The manufacturers have gone to great lengths to design engines to take the lightest oil possible because lighter oil takes less energy to pump and pressurize (more power going to the wheels). However your specific engine builder may not be able to effectively replicate these tolerances, so he may tell you to use 10w30 or straight 30 in the motor that he built for you.
IMO, people think I am crazy for this, but the way I see it I am going to blow my motor before expensive oil has a chance to show me how long it can make the motor last. So I use either Motorcraft oil or walmart special. As long as you are using the correct weight, changing in correct intervals, and changing the filter, you are good to go.
P.S. I do use the new castrol magnetic oil in my wifes car. Cheap oil makes the lifters clack for like 20-30 seconds on startup (only 40k miles btw). This is the only car I have ever had that was bitchy about the quality of oil that went in it.