George.
Larry's reference to a Pirelli "Wet" is actually the name for the Pirelli Intermediate. It will do 4 laps of a dry track (just-for a quick rider).
The Pirelli full wet is known as the "Rain".
Easy to confuse but they are the brand names printed on the sidewalls.

I have ridden the Manx a few times and personally would not race on a proper wet road as we had this year. What baffled me was why the races ran at the time when in some cases a delay of a couple of hours would see improved conditions. I think the idea of a riders "straw poll" isn't a bad idea either.
Rich