One of the aircraft I rent is a Piper Warrior (PA-28) with a Lycoming O-320-D2A. I accidentally put regular oil in it today rather than mineral oil. The engine had run 20 or so hours since its overhaul when I made this mistake. From my research on this model engine, you are supposed to use mineral oil for the first 5o hours.
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I wouldn't worry about it.
The use of mineral oil is mostly about breaking in the piston ring/cylinder bore interface, where the rings wear down the sharp edges of the honed bore surface, at the microscopic level, to create the ideal surface with grooves that retain oil while having flattened tops to make a smooth surface for the ring to slide over. The cylinder bore wears faster using an oil without all the ashless dispersant and anti-wear additives.
The engine will have high oil consumption until the rings have worn the bore surface the desired amount, and a drop in oil consumption to normal levels is the main sign that break-in is done. On O-320s, around a quart in 10-15 hours is typical for a broken in engine.
By 20 hours, much of the break-in process is complete and while I wouldn't switch completely to AD oil, and adding one quart of AD oil, say one 7th of the total of the mineral in the sump, might delay completion of the break-in some measurable amount in a controlled experiment, but nobody in the practical world is going to notice any difference.
