Curiously, a 1980 PA31-325 I've been flying &/or managing since 2007 has a manufacturer POH that includes specific instructions on operating LOP. It even includes instructions on how to achieve LOP if temp limits are reached prior to LOP.
And that's using the original single point EGT. Not that I was comfortable with not knowing if every cylinder was sufficiently LOP, so the only time I ever did it was after I took over managing it to demonstrate to the owner the benefits of LOP and explain the need for an EDM to ensure *every* cylinder is sufficiently LOP. A few months later at its annual, one EDM fitted and LOP ever since.
BTW, that EDM made its purchase price back in a single year of operation in fuel saving. If you include install costs, then that first year still made good due to an injector line failure that year that the EDM helped me to decide to return to the maintenance base instead of landing at the nearest aerodrome - and then having to deal with remote maintenance & a stranded plane & pax.
Last edited by Tinstaafl; 18th Feb 2016 at 03:01.