Plutonium is already considered by IC² (and in RL) a RTG fuel, but there is two more elements which should be added as RTG fuel.
I don't know if RTG fuel is going to deplete over time on IC², but if it is going to, it should be somewhat close to real life half-life times.
Plutonium 238 has a half-life of 87.7 years (MC time converted is roughly 444 days)
Americium 241, small amounts are found on nuclear waste (depleted fuel cell + centrifuge outputs tiny piece of americium 241).
Its generation power is about 1/4 of plutonium.
Americium 241 has a half-life of 432.6 years (MC time converted is roughly 2194 days)
Strontium 90, similar as americium, is found on nuclear waste (depleted fuel cell + centrifuge outputs piece of strontium 90)
Its generation power is similar to plutonium.
Strontium 90 has a half-life of 28.8 years (MC time converted is roughly 146 days)
Fancy thing is that RTG fuels lasts so long that it wouldn't be worth coding a duration time.