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Variant

Oil and water

The layers leave a clue.

Normal sudoku rules apply. Cages contain oil (one parity), water (the other), or both. Within a cage, each row contains only oil or only water. Oil floats on water: no water row can sit above an oil row in the same cage. Digits still add to the cage’s total. Each case’s brief tells you which parity is oil and gives the exact rules.

102413
Oil above. Water below. Four cells from a 9×9 grid. Here, oil is even: 2 and 4 above odd water, 1 and 3. Each row is one kind. The four digits add to 10.
An example case Oil-n-Water Quads

11 open cases