Many, many times the ABC/USBC plays like a reverse block, but you kind of have to be prepared for anything because almost everything than can affect the lanes has at one time of day, at one time of the year in one part of the country.
A friend, with medium speed and medium revs recently told ne that he has used a Lane #1 XXXL as his strike ball for 2 of the 3 events 2 years ago. I've seen it dry but never that dry.
Like NOTHUMB says, a lot, A LOT depends on your style.
I'd definitely suggest, if you have TRUE medium revs, a medium light oil ball with extra length and extra backend for playing a 4th arrow shot that must go no further right than the 10 board all the way down the lanes. Very often, but not always, the out of bounds begins, for the right side, at the 10 board. SO, you taregt the 12 board as a breakpoint, giving yourself a margin of error of 2 boards. (After all most of us are not PBA quality consistent.) Where you target at the arrows depends on the ball and your delivery.
I'd also suggest a dry lanes ball and a meidum-heavy oil ball, JUST IN CASE, both drilled for control. I've hardly ever seen wide open shots like we get in normal house leagues.
If you intend to bowl the BJI or the BTM tourney, you might want to bring a hevay oil ball. SOmetimes they oil heavy; sometimes you get dry-ish. Also a crap shoot.
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"...for advice is a dangerous gift, even from the wise to the wise...."
J. R. R. Tolkien