I had a dpms 308B with an 18" bull barrel with a 1 in 10. 168gr bullets are going to be great. I'd be willing to bet you hit your sweet spot within a .5gr of 43.0gr of varget.
But there isn't a bullet that won't work in a 1 in 10, that I've ever tried. One of the few .308" hunting loads I don't dislike is a 130 TSX in a 300 WSM. It does well a long way out.
I would certainly agree with those that say that every rifle is different. I have a Savage 1:10, with 24" barrel that will shoot anything from 168 to 208, and shoot it well.
However, I have a Rem 700 with 20" barrel, also a 1:10 that doesn't seem to like anything over a 178 gr AMAX. It will shoot 155 gr Sierra Palmas 1/2 MOA all day long.
I guess the answer of "what weight bullet" will be answered by time and trial.
The barrel should shoot just about anything from 125 to 220 grain, some better, some worse. At 18 inches I'm guessing the rifle is in the 6 pound range so lighter bullets will be a lot more fun to shoot.
For "paper critters" id stay in the 150 to 175 grain bullets. If your shooting "fur" I'd recommend 165 to 180 grains but you "could" go all the way to 220 grain round nose. I don't know why you'd want to do that unless you're hunting elephant or something like it because it'll probably kick like a mule.