Question on Wet Weather

I will cut to the chase.

Can you modify GP4 so that even though you have a wet race and the graphics that go along with it, human and AI cars behave as if it were a dry race?

I realise you can adjust grip level for a track through CMagic which i know how to do, yet it seems to be a generic grip level and not one for a dry and wet race.

Have trolled the internet and the FAQ's but nothing quite specifically answers the above.

Thanks,

Slasher.