Refer to this link for info on diffs and how they work - http://www.rctek.com/technical/differentials/index.php

I have not run an adjustable diff in my 2WD Harm Bx1 off roader but as an alternative I used a heavy weight silicon diff oil (I think it was 500,000 weight) and it did improve the traction on loose dirt tracks. However as the diff was not sealed the oil would eventually leak out.

I have found that using different weight (and amounts) of silicon diff oil in the diffs of 4WD largescale on and off roaders does make a significant difference to the handling and especially the front end traction.

Clive