Wrong about the oil: Grease. Looks to be a normal axel grease applied to the spring, to keep it from squeaking against the spring housing, and probably to lube the lower spring housing bushing. The felt under the base exists to stop the downward flow of of liquified grease that escapes through the spring housing bushing, and probably to help keep the goo away from the rubber stop, just underneath. Of course with a bit of time the goo infiltrates the felt and reaches the rubber. The rubber gets soft enough to allow the locknut on the shaft to hit the base, creating a thunk, as I said before.
I disassembled , put some new grease on the outside of the spring, cleaned the bits of felt out of all the places it had wandered, cut a new 5mm thick felt, hole punched it, and reassembled everything, using a 14mm wrench to hold the upper chain block in place while cinching down on the locknut with a 10mm (if you don't center the block , the chain and pedal will be adding a torsion force to the downward motion of the shaft, holding it more tightly against the spring housing lower bushing). Works just like new. Not sure what was slowing the return; maybe bits of felt getting caught in the passages, but whatever it was has stopped.
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