Step 1: check with a mechanical pressure gauge. The instrument cluster readings aren't too accurate, they are more like: "low, medium and high." NOT "20" on the gauge = 20 "psi."
Step 2: go from there.
Mileage is not excessive, but do you know the history of the vehicle?
Wouldn't hurt to check for proper oil level. I had this wacky condition on my K3500 where when I took off, the oil pressure gauge would dip to zero momentarily, then bounce back up to normal. Turned out the crankcase was low on oil. I hardly drive that truck and it leaks everywhere underneath. I just didn't think it leaked that much engine oil.
If oil pressure is truly low, drop the pan and start looking for glitter and sparkles. Also cut open the oil filter and inspect the filter media. This may be your case since you mentioned you already replaced the oil pressure sender.
I ran the 4.3 on my 99 Silverado with low oil pressure for over 1 year and probably more than 10k miles. After I concluded I had truly low oil pressure, I decided to switch to 10w-50 and kept going. Pressure increased another few psi, enough to get me out of the "check gauges" message at idle. I didn't have money for a rebuild and that was my only vehicle at the time. Engine still ran fine until I retired it.
Good luck.
