Drain the guavas, reserving half the syrup. Chop fruit and add to a saucepan.
Add orange juice + zest, sugar, lemon juice, and half the reserved syrup.
Stir over medium heat until the sugar dissolves. Bring to a gentle boil.
Reduce heat and simmer for ±25–30 minutes, stirring often, until thickened.
For a smoother texture, mash lightly as it cooks.
Add pectin in the last 5 minutes if using, and simmer until set.
Spoon into sterilised jars, let cool, and refrigerate.