I have had that stuff hanging out at the oil fill hole after 2 or three hours at high way speed. I was using a 195 degree thermostat at the time. I'm not sure how cold you get down there, but it was wintertime when I was driving through nevada and this is when I noticed this. All I know is it takes a long time to get rid of it all, but mostly it is just hanging out where there isn't a lot of engine heat, the important part clears up pretty quickly. But it was bad enough that when I took off the valve cover there were flakes of rust that had fell off the inside of the valve cover. Which is mostly why I got an aluminum valve cover, it isn't really affected by the moisture. Yes it will corrode, but I don't think it will be to the extent of flaking pieces off into the engine.
