I did this on Tuesday or Wednesday and I noticed a huge difference in the oil gauge. Before i changed the oil, the oil reading would be on 20psi on idle. After I changed the oil and put the new in, the gauge reads 40 psi at idle. Is that good or bad?

Sounds like normal pressure. Why it was 20 before new oil, I don't know. Did you change the filter?

that would be good. 40 psi is about where it needs to be. oil has a tendency to thin out as it gets old. i noticed this same thing on all of my cars.

Oh yeah. I was stuck on gunk in the oil making it thicker, I forgot about oil losing viscosity.

yep, I change the oil and the oil filter.

You were probably well overdue, but you're golden now.

Not only will oil lose viscosity but it will dilute with unburned fuel and condensation.
The pressure will also drop a bit from restriction of a dirty filter.

you should change it again sooner than normal, say within 2000 miles, just to get the rest of the crap out of there, can prevent knocking and heavy engine wear in the future..my 02 cents

yes sounds like your oil was way past due for a change as your oil was thinning out,
change oil every 3000 MI for dino or every 5000 MI for synthetic.

coulld have been low on oil