Yet another oil discussion -- Best oil for Me.

[HAULN-SS]

So, just to jump in on this discussion to clear things up - why does it seem like bigger engines call for stuff liek 10W-40 (read it on the internet) and smaller engines for 5W-30? Is that just something to do with power conversation? I'm assuming thinner oils dont rob as much power, and that counts for more on smaller engines?