Ok let me explain in this super detail for you, because to be honest i think glight has given the wrong information here, no offense
The Intermediate tyre on F1 2010 is completely useless! Why do you ask? Here comes the super detail from me...
First of all the game seems to have made the Intermediates to be used in very light rain, however when the game begins to give you rain, it seems the Dry tyres work exactly the same in the light rain, therefore it is completely detrimental to pit in for Inters.
When the track becomes wet, to the point that the car is splashing out water from all sides, and the dry tyres begin to drop off in performance, at this point in time, pitting for the Inters is completely useless, as it is now too wet.
What Codemasters did... was they made the Dry too grippy in light rain, the transition period from Dry, to Light Rain, to Moderate Rain and ending with Heavy Wet Rain can happen in a very short time, and from my experience i have never seen it where the track goes from Dry > Light Rain, and then back to Dry again, it will always proceed to at least Moderate Rain, where the Intermediates become useless.
So my advice to you is, completely ignore the Intermediate tyre, even if it puts you on Inters at the start of a session, change to Wets immediately, the Wet tyres are very grippy in Moderate rain, whereas the Inters are just completely useless, if your in Heavy wet conditions, and you find the car is unstable, change to Cruise Map setting for a while, until the weather eases off a little bit and the tyres begin to grip the track more.
Codemasters failed with the Intermediates in this game, i can tell you from experience racing awesome drivers in my championship, any time they have gone to pit for Inters they have come back in 3 laps later at the most, when they realized just how much time they were losing.