Dear Andrew,

1) It is not a hard and fast (thumb) rule to determine how much inoculant of A. niger should be used to treat oil contaminated soil. The amount of inoculant is depends upon several factors and the major one is environmental conditions i.e. type of environment near to contaminated soil, soil texture, type of oil etc. Other factors like microbial strain, its efficiency to degrade pollutant etc. 

Studies well reported the efficacy of Aspergillus in bioremediation and can use the spores as inoculant in the range 1-5% (spore count 105 to 107 per ml or g).

2) Spore inoculation is the preferred method but you can also use some inert carriers such as peat, lignite or charcoal for inoculation (same as biofertilizers).

3) The quantity of spores can be measured by spore counter (Haemocytometer or Improved Neubauer Haemocytometer).

 I advised you to search for relevant research papers on internet. Lot of research material is available there. 

Regards,

Kunal

Incentivated crude oil is a viable low cost technique if you do consider the relevant issues. 

how old is your spill

Thanks Kunal.

I agree with Borys Didyk answers ...good luck.