Snow Tech had a really good article on plug reading and piston wash earlier this winter,Read it.
Oils play a very big roll as well as the type gasoline you use !
In the article it said to look at the bend of the electro as well as the positive ground and porcelain to read the plugs, If the plugs are white at the bend, positive ground and porcelain and white or gray on the piston top - YOUR TO LEAN.
look for brown, tan or a redish brown on your plugs and brown color on the piston tops, with only a few areas that have clear spots (transfer ports) Black piston tops could be either to rich (if wet) or to rich in some areas and to lean in another area and burning - or the oil just can't take the heat, period, or the oil is so tough it can't be burnt and should only be used on race engines.