1967 Chevy CST aka "the loud truck"

[67chevy1series]

Well replaced the oil pan gasket and I found the problem... The rear oil pan gasket section (the one in the 4pc gasket kit) well the old original on was still there... So I cleaned all the surfaces took that old piece of gasket off and then installed new 1pc oil pan gasket and then filled it with oil checked the oil level and it said I was full no leaks so I started it up and watched my guage... And the needle went to the 1st line out of the 3 in the middle and as I turned around in the court I noticed the oil pressure dropped to the L so I shut off the truck put it in neutral and rolled to a stop got out checked the oil level and it was good still... ??? So I got in started it back up and the guage did the same thing.. So I proceeded to drive home and throughout the drive when I would come to a stop and idle it would drop to the L and then it would go right back up to the first mark and on sharp turns the oil pressure would go down. Throughout the whole drive the oil pressure never rose above the middle line.. So my first question what in the hell are those lines? Because any type of things i try and research to find out what could be wrong they all talk about 30psi or 10psi and it's getting a little irritating because I honestly don't know what those lines represent all I've ever been told about these trucks is to make sure it stays within or at the 3 middle lines if it goes over the 3rd one shut it off if it goes below the 1st one shut engine off so I'm guessing it would be 0\ 10\ 20| 30/ 40/?