Low oil pressure.

[get me on the road]

My dash gauge says I have about 20-23. When I got the truck this summer it would run at about 40 while driveing, then maybe 25 at red lights. I checked the oil, it was down about a 1/4 quart, filled it, still reads 20-23 cold or warm. The only oil leak is the valve stem seals, so it smokes when you start it, but thats it. I'm pretty sure it dropped yesterday, but no ealier than wednesday. I am hoping its a oil pressure sending unit going bad. Is there anyway to test it, and the dash gauge? Any thoughts or comments, as always are appreciated. Oh one more thing, on wednesday I started driveing it in drive around town 48 mph and under, then pop it in to overdrive at 49 mph and above. I started doing that because of a thread I read at this site.