Answer will depend on the rules, but for dealer S17,
Always split 8s vs an Ace.
8s vs 10 will split unless the count goes positive, then you'll tend to surrender.
8s vs 9 will almost always split, although there are some very high counts which may surrender (but these are rare). This is for both DAS and noDAS but DAS requires a higher count to surr then noDAS.
Sims have been run to substantiate my response. Hope this helps.