Merriam-Webster has this to say:-
Farther and further have been used more or less interchangeably throughout most of their history, but currently they are showing signs of diverging. As adverbs they continue to be used interchangeably whenever spatial, temporal, or metaphorical distance is involved. But where there is no notion of distance, further is usedHmmm. I thought I'd check out what Language Log had to say on the matter but it seems the subject is too dull even for them.
. Further is also used as a sentence modifier , but farther is not. A polarizing process appears to be taking place in their adjective use. Farther is taking over the meaning of distance (the farther shore) and further the meaning of addition (needed no further invitation).