What kind of plums should I use in this recipe?

You can use red or black plums for this recipe. These are the round plums you will most likely find at the grocery store. Damson plums are a sub-species of plum that is smaller and more oblong in shape. These, along with Italian plums, can also be used.

Do I need to use pectin in plum jam?

Stone fruits generally do not have enough pectin to allow jams to set properly. Note that Damson plums and Italian plums have less natural pectin. If you do not add the pectin, make sure to cook the jam for longer, until it is thick and at a temperature of 220°F.