^Exactly. "It is designed to be at the right tolerance when warm." The engine when dirty has to work harder at keeping the engine warm, and not HOT. Side Note: Even if you are in frozen over Canada (while I am almost in the tropics), the climate doesn't matter(to an extent), once an engine gets hot then that's what the cooling system is for. The cooling system does a better job when it's not fighting trapped heat. Does an engine every overheat? Yes, it does. Why? Many reasons, being dirty affects those reasons and makes it more prone. Keep it clean. This is mostly likely what matt, above, is saying... not to beat a dead horse.
