The PA31 have bladder fuel tanks, as quoted in the POH it is advisable to leave a little amount in the aux tanks, and I guess 10lt could be enough.
Never leave the fuel cells completely dry, or the cell inner liners may dry out and crack, permitting fuel to diffuse through the walls of the cell after refueling. If the cell is to be left empty for a week or more, spray the inner liner with a light coat of engine oil.
Cheers,
Capt EFIS.