They were using Plutonium 238, which is the most common RTG fuel. The Wikipedia page on RTGs has a fairly comprehensive overview of the possible candidate fuels[1]; one advantage of 238Pu is that it requires the least amount of shielding, an important consideration when launching things into space.
Pages 101-105 of the termination report I linked to above have the power output curves for the various RTGs, showing the decay curve (1 lunation ~= 1 month).
