This is my story-

I put slick in when I bought my van at 87,000.  Put it in again at 160,00.
Thinking about it again at 220,000.  I have done nothing else to the engine,
other that extraneos stuff (water pump et el.)  So that is at the very least
130,000 miles by me with no valve work, (even adjustments)  heads, rings,
NADA.   Never needed to... always ran fine and at 21 MPG.

Now however I do have a prob.   Lifters knock every 2-300 miles and I have
to change oil to correct problem (gets very thin).  I have not been able to
determine problem.  Could slick 50 have done this?  I suspect some other
harder to figure culprit.  Aux air regulator?  Intake Boot?  Somewhere
Sensor? Computer?  If only it were easy.

All in all however, I would say I had a good expierience.  Fortunatly I
don't drive it more than 20 miles per week now.

Kurt