You will only cook the center diff if traction goes up "and you have too much power". Keep in mind that one of the things a diff does is, get rid of unwanted power from tires that are hooked up. Oil thats too heavy can interfere with this diffing of unwanted power away, causing a loose and hard to drive buggy. There is nothing that says you cant mix 5k and 7k 50/50 to get in between for the perfect combo. I run a modded C6 in my buggy, and had 7K in to start (it was a bit loose), then tried 5k (ballooned the front tires a bit too much), but ended up with a 50/50 mix and its perfect. Now I just fill exactly the same each time to keep the consistency good. If I go to a high traction track, I go to 7k in the center.
