I was doing an oil change to my car and did not realize I did not have enough oil for the engine, only a quart. I added what was left and did a 20 min trip to a gas station to buy oil so I am wondering how much damage did I do on my engine if any? The car in question is 1993 Mazda Miata, it has an oil pressure gauge which did not drop much more than 30 during the trip also I think the oil light is disfunctional.
Very risky move… It could have turned out very badly.
Off hand, it's hard to say, but if the gauge is telling the truth and really did stay around 30 psi, you may be OK. However, some gauges have a restriction on the sender inlet to damp out short term pressure fluctuations, so the gauge may be unrealistically optimistic about the instantaneous oil pressure.
With only a quart of oil, between the oil in the filter and the oil in the oil passages, you have precious little oil in the sump at the bottom of the oil pan to feed the oil pump. So the oil pump may have been pulling in air from time to time, especially as you went around corners or accelerated/decelerated. Hopefully you were driving conservatively.
On the other hand, the oil film that the engine depends on doesn't break down immediately when oil pressure drops, so you may have gotten away with it. If you want some reassurance doing an oil analysis or two would probably give you an idea of the condition of the bearings and other internal parts.