Which diff oil to use in my s400 geared diff

[Hoggy]

Hi, guys

Does anybody know what is the best weight diff oil to use in my geared diff in my serpent s400 touring car,I will be running on carpet indoors and the outdoor tarmac track at halifax ,I run a fixed spool at the front.

I dont know which weight of oil is in the diff at the moment because i bought the car second hand but the problem is at the moment the back end is always trying to break away,Which means the diff oil is too heavy.

Any suggestions are greatly appreciated.

Cheers