After considering all the methods suggested here and trawling the 'net, I decided to try naphtha and diatomaceous earth.
I couldn't find the diatomaceous earth (comes from swimming pool supply companies and there isn't one in walking distance from Cowes) so I used kitty litter.
I went to bed
last night sure that I would be faced with the extremely unpleasant choice of living with hideous spotted decks, or shelling out God knows how much on replacing the decks aft. The problem -- it seemed to me -- was that it was used
motor oil, full of carbon black from the diesel
. Carbon black makes an excellent dye. Argh!
So I mixed naphtha with kitty litter and slathered it on a small area as an experiment
. Left it over night.
Today the test area looks great -- silver colored like the rest of the deck
, with no trace of the spot! Yes!!
Now I will have my guilty crewman find some diatomaceous earth and do it all properly.
I am sure this can't be good for the teak -- sucking out all the oil and drying it out, I'm sure. But I really can't just leave it -- it utterly ruins the appearance of the deck
Anyway -- for the greater good -- here is a method which works.