" jls" wrote in message
...
Not simple. Questions abound: depending on type engine, type
induction,
whether turbocharged, field altitude, density altitude, formula gasoline
(Some people love autogas), and much more. For example if your plugs
aren't gapped correctly you're going to have a serious problem sooner or
later.

I saw an O-235's plugs recently and couldn't believe the lead deposits and
fouling. Don't know if they were gapped properly but the spaces inside
the
plugs between the ceramic and the inside diameter of the plugs were filled
with hard gray deposits.

http://www.avweb.com/news/columns/182155-1.html (About half way down the
page).