I have used motor flush with what i believe to be great success. the post preceeding mine has its merits, but motor flush can be used if the history of the motor is known and it has had regular oil changes. i just used motor flush in my 85 300dt with 150k and after i drained the oil with the flush in it, i filled it with some 5-30, ran it for 10 minutes at 1500 rpm, then drained and changed the filter, and added 5-40 synthetic rotella. i believe that if you flush the motor immediately after with straight oil then using flush on a well maintained motor can only help. i know, as do 2 bystanders, that my car sounded smoother after i was done. now the question is, was it because of hte motor flush or because of the synthetic oil? i've used flush every 12k and have had good success on numerous motors, to include 2 nissan maxima diesels that i had back in college. i got over 200k out of both of those motors with good compression and easy starts down to 5 degrees f upstate new york. i sold both of those cars in good running order and never did any internal motor repairs. motor flush is good in my opinion if used right.