Sporadic Low Oil Pressure Light

I just inherited my father's car. Had to drive to North Carolina to pick up from my brother, and on drive back to Atlanta it drove and handled beautifully. When I picked up car, it only had 76,777 miles! Only problem was that twice I had to hit my brakes pretty hard due to traffic and the low oil pressure light came on for a few seconds. Not talking about tire squalling hard, but still a lot harder than normal. Since I have been back, I also had low oil pressure light come on twice more, but only when I had been at stop light for three or four minutes, idling with transmission in gear. As soon as I gave it gas, light went off. Took it to a national chain store and they said it could be caused by any of several problems, including main bearings going out, faulty oil pressure switch, or since my brother just had oil changed two weeks before I picked it up, cheap oil, or wrong oil. Tag left on windshield indicated they used 5W30, but didn't indicate brand. They replaced oil pressure switch, flushed engine and replaced oil, and also added “Lucas” additive, then said that I needed to use “Lucas” additive from now on when I changed oil. Mechanic tried for 10 minutes to get oil pressure light to come back on, but it did not, however, the mechanic also said he used to work for Chevy dealer as mechanic and that the 3.4 V6 was very unreliable and had history of problems. He also said he thought he heard knock coming from bottom of engine, and thought main bearings were going out. Now I am very worried about driving it. Low oil pressure light has not come back on, and neither my wife nor I hear any knocking noise. Does any of this make sense, and should I be worried about driving the Impala? Since it is so much larger and more comfortable than either my 2006 Suzuki Aerio or my wife's 2010 Hyundai Accent she wants us to use it to drive to Florida for vacation in July.