Hi mate,
ive never raced on dirt but would imagine a good set up would be similar to what we use at Worksop which is quite slippy.
Some good sets ups can be found here:
http://www.team-durango.com/pdf/setu...n_22_11_09.pdf
http://www.team-durango.com/pdf/setu...07_02_2010.pdf
http://www.team-durango.com/pdf/setu...20_12_2009.pdf
http://www.team-durango.com/pdf/setu...28_02_2010.pdf
I would say you want to be using thinner diff oils on loose dirt as this will help gain side bite and generate more grip.
Maybe try 3k in the rear ah 5k in the front.
Hope this helps a little.
Craig Collinson