If your main bearings are bad or your crank or block is worn, you seep too much oil and your pressure goes low after 10-20 minutes. Add some Lucas stabilizer. If this works it's temporary. You'll need to rebuild your engine.
Also, Pick up an oil pressure gage and measure the pressure at the sensor location on the cylinder head. If the pressure really is low, see what I said above. It's very common for the sensors to go bad, but an oil pressure gage is only $10 and it's best to know for sure what the problem is, especially if it's low oil pressure.