synthetic
 

Ive changed my trucks oil to synthetic, ever since I have noticed the oil pressure gauge reading low at idle. Whats the deal?

Are you referring to 'low' as being lower than it used to be before the synth switch, or at the bottom of the gauge / needle doesn't move?
If you stick a real mechanical gauge on there and have around 7-10 psi at idle warm and between 36-71 psi at 3500 rpm's, you should be fine. If you don't, it could be the pump is worn, the crank or rod bearings are worn or the oil is too thin for your conditions.

My gauge read lower with 10W30 synthetic vs 10W30 mineral across the temperature spectrum. Oil pump looked new when I had it out a little more than a year ago.

ok thanks. yes it jst reads a bit lower than when i had mineral.

it's not a big deal. you usually have less pressure with synthetic, as it's slightly thinner (despite having the same weight rating). As long as you have oil flowing, it won't be a problem.

My motor on 5w-30 synthetic usually sits just below the 1/4 mark on the stock oil pressure gauge at idle.