Pu-239 will eventually decay to lead (Pb-207). Pu-239 has a half life of 24,110 years after which time half will be U-235.
Pu-240 will eventually decay to lead (Pb-208). Pu-240 has a half life of 6,564 years after which 1/2 will be U-236.
Pu-241 will eventually decay to bismuth (Bi-209). Pu-241 has a 1/2 life of only 14.35 years.
Pu-242 wil eventually decay to lead (Pb-206). Pu-242 has a 1/2 life of 373,300 years. The first step in the decay of Pu-242 is U-238. Pu-242 would have been easily detectable in the Oklo reactor for at least 3 million years.