7k obviously proved to be really good at Worksop but that's on a cork surface, which is fairly loose traction. 7k on Loose traction tracks will help prevent the car from diffing out under power.
On a high grip track you shouldnt really have any issues with the car diffing out which enables you to go much lower on the diff oils. this allows lots of quick rotation which is good on tight technical tracks with plenty of grip.
Diff oils vary from track to track, so best thing to do is have a play around and see what works for you.
