A combination of sweet and tart plums will give a more interesting flavor.
Heat oven to 250 degrees. Cut plums in half lengthwise and remove pits. Cut halves into 2-inch wedges. Place in a large mixing bowl and sprinkle with sugar, salt, pepper, vinegar, and oil. Toss to coat well.
Arrange plums on a parchment-lined baking sheet, cut sides up, about an inch apart. Sprinkle liberally with fresh thyme. Leave to dry in oven 3 hours. Turn off oven, and allow plums to remain inside at least 2 more hours or overnight. Plums will be wrinkled but still quite juicy when done.