Most abacuses are base 5, or more precisely mixed radix (5,2,5,2,...). However simple base-10 abacuses with 10 beads per pole, typically marketed toward children, are very intuitive and easy to learn how to use.
Base 10: adding 8 is equivalent to adding 10 and subtracting 2. Just have a chart of complements base 10.
Base 5: adding 8 is equivalent to adding 5 and adding 3. Adding 5 is equivalent to adding 10 and subtracting 5. Adding 3 is equivalent to adding 5 and subtracting 2.
Color the beads on a base-10 abacus in a way that it's easy to count how many you are moving and how many are left. I have used one that had just the 5th and 6th beads differently colored.
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