Oil sludge could caked over the time and could obstructed the oil passages of the oil pressure switch. I pasted this information from another forum site and i hope that give you the right idea.
On a badly maintained engine, there's probably already too many deposits for the flushing chemicals to completely remove. But the real danger here is that the chemicals will dislodge enough crud that it will block the very small passages in hydraulic lifters or even clog up the fine mesh oil pump pickup screens. On such engines, you're better off changing the oil frequently, say around every 2,000 miles, so that the natural cleaning properties of the oil will reduce, somewhat, accumulated deposits.
Read more: How to Perform an Engine Flush in Your Car | eHow.com http://www.ehow.com/.../...ne-flush-car.html