I just looked at a used 2002 Ford Focus 2.0L 4 cylinder that has 169,000 miles on it. The seller is the third owner and has had if for the past 39,000 miles and has run nothing but Amsoil in it. I peeked into the oil filler hole and was stunned. It looked like a brand new engine inside. I did not see any crud, staining or other, just clean shiny metal. I have never seen anything like it. Usually there is a yellowish layer of varnish inside an engine fill hole along with the usual blackish coating on many of the surfaces.