Running an engine for 20 hours on a bench won't run it in. Not unless you have a dyno to load the engine of course...
In my opinion starting it occasionally to move it and enjoy the experience shouldn't do any harm because in 'normal' use the various pastes and additives will be wiped and washed off the cam and other critical surfaces in minutes so they amalgamate with the engine oil pretty quickly in any case. The important thing is to be sure you have oil circulation, take the filler cap off and check the rocker shaft is oozing and splashing in oil.
An engine needs a load to run it in and make the rings 'work' I have had several large engines, among them a Cat D4 and Volvo 12 litre truck, which when started smoked like crazy and would do so all day if left just ticking over, within a minute of making them work the exhausts were as clean as a whistle and stayed like for the whole shift.
You have to work an engine to get it running right, that's what it's designed for and it takes at least 15 to 20 miles to get it warmed up properly. One reason why racing car engines wear out so quickly, they don't have time for the working temperature to soak right through the entire engine and get the tolerances right. Formula 1 engines are heated overnight so when they are started they are on song from the start, else they would seize and fly apart before the first corner.
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