The 7:38 time was on "cut slicks", according to the Chief Engineer in this article:
http://www.pistonheads.co.uk/news/de...?storyId=17066
But the track had some "semi-wet" areas.
This seems to be the only source reporting "cut-slicks" so it could be a true cut racing slick, could be "DOT" race rubber, or it could be a language translation issue, as the Chief Engineer goes on to explain they weren't interested in race slick times.
Even among cut slicks and street-legal race rubber, you can buy everything form a qualifying tire good for 1 hard lap to an endurance type tire harder than some street legal tires. I don't think we'll ever know exactly which tire everyone uses unless it's an official Sport-auto time where they specify OEM rubber