They are succeptible to a whole lot more than that. Dropped valve guides were common in the earlier years, and valve guide wear is still a problem. At about 90,000km the heads on my New Yorker were replaced under warranty due to a couple dropped guides. At about 190,000 I had to replace the heads again due to worn guides causing oil consumption and killing the cat. New heads and cat passed emissions just fine - and again this year at 240,000km. Darn MitsuShitty engines!!!