Low Oil Pressure 2003 Impala

I HAVE A 2003 IMPALA 3.4L. The manual says that the oil pressure should be 60 psi at 1800 rpm. I removed the oil pressure switch and screwed in the hose for the guage. Is this the right place to check the oil pressure. The dealer has a device that the oil filter is removed and a device is fitted in place to measure the oil pressure. Anyway I measured 40 psi at idle going to 48 at aprox 1800, cold and 38 psi warmed up going to aprox the same as cold. what do you think ?
so how low is low oil pressure ? how low before engine damage ?

In most GM's, the standard oil pressure is around 40 psi. I'm not quite sure of your impala, but in trailblazers, intrigues, and impala's with the 3.8, the needle starts a little below 40 psi when warm, and then when revved, can go up, sometimes as little as 0 or as much as 10-15 psi.