Sorveltaja,
The late Bob Shores used to use a petrol/WD40 mixture for running his i/c engines, and I followed his lead on the last one I built, a Westbury Whippet.
For running in I used about 6% mix and then 3% to 4% for normal running.
The engine stayed wonderfully clean on the inside and out, and I never had any trouble with plugs oiling up.
Bogs