your question about will a blue dot slide all the way down in the dry - it depends on just how dry is dry.
If the lane is truly dry (1970s 80s dry), the ball will grab and hook. No question. If you're talking too dry for extreme reactive equipment, then the answer is probably not - it'll slide, because even though there's too little oil for the heavy hooking balls, there's still too much for polyester.
I have seen some awesome scores shot with blue dots recently. Do I use one for a strike ball? No. Have I used my spare ball for a strike ball occasionally? Probably once or twice a year. Not really a big enough deal for me to worry about, but the guys with lots and lots of hand, and no knowledge to control it - yep, they use spare balls for strike balls once in a while.
There are two statements you made that concern me. The first is the statement that you only use the 10 board as a target now. The best bowlers will adjust not just their foot position, but their target on the lane based on the changing oil patterns on the lane. Sometimes I'll target outside5 board, sometimes as deep as 25 board at the arrows. It just depends on where the oil has moved to, and where I need to play to get the best carry(in the oil, or in the dry).
The second concern is about the spare shooting. The 3-6-9 spare system has worked from the days of black rubber equipment to today. I use it every day. However, I don't use it the same way with my spare ball, and depending on how high the oil crown in the middle of the lane, it may be the 2-4-6, or the 4-8-12. All the systems out there are proven, but take some knowledge to apply correctly. If you follow any of these blindly, and don't take what you see into consideration, you will not improve. In other words - if you use 3-6-9 for adjustments on right side spares, and the ball is consistently not getting far enough right (I'm talking to you as a lefty here), MOVE!! MOVE LEFT!! You may need to adjust 4-8-12 on that particular pattern. Don't assume it's you, or your equipment...
Any way - hope these help.
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One advantage of bowling over golf
is that you seldom lose the ball.