Ok it looks like the post went a little of track to what I want it to be about.
So originally I thought I had a ball diff to put in the car, I didn't really want to take it out of the 4wd and thought I may be able to get a gear diff to be good on the slippy stuf (Worksop).
So my take on the difference was that a gear diff running low oil would be similar to a ball diff, coming from 1/8th nitro I don't really understand /like ball diffs, that's mainly because I don't really know how to adjust them correctly I suspose. Obiouvsly when I found out the diff I had in my K1 wasn't correct I had to stick with the gear diff.
Now reading the above it looks like my thought was wrong on the oil selection in gear diff's. My understand was that the thinner the oil the less drive I would get, hence going lower on oil, also taking a set of gears out out would make it smoother, less drive again.
So it looks like all my thoughts were incorrect, I didn't realise a ball diff would be run loose on high grip, if I had left my normal 15k oil in for Worksop it would have been better lol.
See, this is what threads are all about, learning
Thanks
Ross