Half a gear diff better ?

[Bells]

Tonight I decided to rebuild my sons gear diff and try a different weight oil in it , since buying it in December as a rtr we have made a few mods to it mainly mm4
It has always handled better than my kit built 210 on slipy floors ie cullingworth and batley indoors , now we are outside its the reverse my car is a lot better , my gear diff has 3000 in it and my sons has no oil just a bit of black grease and only half of would should be there ! See pic

I will be trying this on my car the next time I can get to cullingworth to race ,
Anyone got any ideas as to why this would be better on slipy floors ?