Oil Guage and Tachometer

My first question is going to sound really dumb, but what does the oil guage in the dash do? Does it measure how much oil there is in the engine? Something like an electronic dipstick? Reason I ask is because I just had my truck lubed and oiled and the guage is still reading near the low end, that why im wondering if it does something other than what I thought it did.

My other question is, how hard is it to change a tachometer? Mine is going nuts when im idleing. Its jumping from 0 to 3 and back down and all over the place.

Thanks for the help in advance

it measures oil pressure,,,,in psi. if its reading low, it means two things...neither of them good. one, is low oil level, or a weak pump. if the mileage in the engine high, the pump output is low, or excessive bearing clearances. check the oil level on the dip-stick first, and if its up where it should be, then look at replacing the oil pump.

Sounds like He's right on, the only stupid question is the one You don't ask, then kick yourself on the side of the road with Your thumb up in the air.