I think the thing with Lynch is that you really force the defense to stay near the line. Yeah, Turbin runs really well, but he doesn't have the same effect on the defenders' minds. Turbin didn't actually do all that well last week with 1.5 YPC, so that probably had something to do with it too. If we had gotten up more my guess is that we would have seen more of him, but with a 6 point lead I can understand wanting your best player on the field.
Where the heck do I go if I want to find out how many downs player A played versus player B?
Oh, stats are fun. In week five, Lynch had 50 snaps with 23 utilized plays (carries + targets) and Turbin had 12 snaps with 6 utilized.
Over the entire season, Lynch has 244 snaps (122 utilized) and Turbin has 56 snaps (24 utilized).
One thing I think is interesting about Turbin's "utilized" stat is that it's just another thing that shows that the coaches have no problem using him the same way they use Lynch.
Sources: Snap counts