What about something mild like Swarfega to start with then wash with water. It may shift the bulk of the stain leaving marks that will fade with time. Or white spirit would maybe maybe help as well but may just spread the stain. There are drive cleaners that reputedly remove oil from driveways that may also be useful to try.
A trick I learnt some years ago from a furniture restorer is to lay the duckboards flat. Coat the affected area with methylated spirit and light. It does not burn the wood, but the heat draws the oil out of the wood. You may need to repeat several times.
Are your duckboards teak? Is putting solvents on the wood a good idea as that may remove some of the valuable oils that help preserve teak..
My friend and i had to change the hydraulic steering on his rustler last week.. We spilt a fair bit of oil on the teak cockpit decking.. He didnt care as it wasnt harming the wood and said it would soon wear off !
No idea if it will work on wood but talcum / baby powder is great at getting oil and grease off leather, paper and clothes. Ive also seen reference to it being used on stone and concrete. You just sprinkle it on and leave it for a few hours / overnight to draw out the oil. Might be worth a try.