for the last couple of years, I have been able to tell when it was time to change the oil because right between 4 and 5k miles the oil pressure light would start coming on while at idle once the engine was running hot. I changed the oil and filter this weekend which usually solves the problem, but the oil light still comes on at idle after the engine warms up. Normally by the time I do the oil change, I am down about a quart. Is the oil pressure sensor(?) bad maybe? or do I have some other problem.

 

I would not be happy with the oil you are using if it's losing its viscosity like that. There are some bad oils on the shelves, believe it or not, some may even be used oil, unrefined. Find out more here: http://www.pqiamerica.com/

For me, no house brands, don't know what additive package they are using nor are their product data sheets readily available. Additive packages make the oil. Major brands for me, Valvoline Premium Conventional 5W-30 is often on sale at Walmart here so that's mostly what I use. It has a cold Pour Point comparable to synthetics.

Your sensor is likely bad, they have a reputation for that. Mine doesn't like very cold weather and needs replacing.