I like to play down an in, and do it well. However, as you probably understand it always becomes necessary to move in as lanes transition. First, more important than what ball to use, is how to make the move. Too many guys just want to move their feet in without bringing their target in as well. You still have to play more out angle, but you don't start standing left and throwing right like a power player. If I move in to 15 board at the arrows, my break point will seldom be outside of 8 or 9 board. If I get to 20 board, my break point will be inside 10 board. I simply don't have enough rev rate to cover a bunch of boards and still have energy left at the pocket.
Turning to the ball, I like strong low rg pearls when I get deep, sometimes I will even throw a strong solid without much surface. The mistake people make is trying to go weaker when they move in. When I am going down and in outside 10 board I will use my smoothest rolling ball tame ball, especially on a house shot. As I move in, I want something that will save energy, but make a strong change of direction down lane to compensate for less entry angle. The only time I go weaker inside is after I have gotten as deep as possible, and I simply cant get anything stronger through the heads.
Also, a lot depends on the length of the oil pattern. On shorter patterns I will find ways to play further out, and stay out with the tame balls. On longer patterns I usually go to the strong pearl right away. I will play relatively direct looking for some early hold area , until the track starts to open up. Then I often don't need to make a ball change as I move in.