I learned something new yesterday.
I never knew that you could use lead and oakum with PVC pipe. On Ask this Old House they were attaching a new PVC drain pipe to the existing cast iron where it went through the foundation. They cleaned out the old lead and oakum, inserted the PVC fitting, stuffed in the oakum and poured in the lead. They said they had to wait at least a half hour for the PVC to cool before continuing the work.
I assume the softening and then hardening of the PVC is what gives the lead the bite it needs to hold.