oil gauge registered very low oil pressure?

I had low oil pressure when running my A/C on the interstate during very hot weather when i come to a stop light my check gauges light came on while in drive, but if i put in neutral the light went off. my original oil gauge registered very low oil pressure. i was using synthetic 5ww-30, i changed the oil to conventional 10w-30 with a high flow oil filter and i had a new heavy duty oil pump put in. i want to put a manual oil pressure gauge using copper tubing is there an oil plug on the side of the motor that i can hook up the mechanical oil pressure gauge to and find out my true oil pressure of my motor. I'm using 1/8 copper tubing this is the only time my oil pressure is really low. I also put a quart of Lucas oil treatment in the new oil and I put it in a new oil pressure sensor. Thanks