Facts as I recall are;
1) Prior to ED/ENG/4 amendment 7 there were NO on condition piston engines. All were specified in ANO's as hard time overhauls.
2) Engine manufacturers recommend fixed times.
3) The airworthiness bulletin has dubious legal ramifications (still checking), to make engine extensions. CAR 42B does and always has had engine checks as part of the system of maintenance and in the case of schedule 5 is the CASA schedule. All piston periodicals I have had anything to do with included spark plugs, oil filters etc. AD/ENG/4 is the instrument that extends hard times to on condition subsequent to condition checks including oil uptake etc.
I fear that to obsolute AD/ENG/4 would take us back to some obscure ANO and fixed hard times between overhauls.
Why not just leave this one alone. I refer you to your own first paragraph of your last post.