Ive done several 420 , 560 sels all ran poorly around 150-200 k miles, they all ran like new after a valve job and new guides, all went over 250,000 miles with no issues, my brother in law has a 420 sel now with over 475,000 miles, all these engines got a valve job new guides and timing chains, oil use was normal after work done, these bottom ends are over built, all engines were cared for and serviced on time