On my 86 Fiero (less than 2 years ago), I drove about 100 miles with it in my oil, and it didnt do any harm. A lot of people advised me against it, but it was okay. I wouldnt recommend going THAT far, but its up to you. The longer it stays in, the cleaner it will get things (which honestly isnt the most important thing in the world), but the higher risk you have of destroying seals and components. What I've been doing lately for oil flushes is getting a product that can be found at any auto retailer called "Motor Flush". Its 1 qt, and it comes in a grey bottle and looks very generic. I do an oil change leaving it a 1/2qt to a qt short, add the flush and let it run for 5 to 10 minutes, drain it and fill it with some good synthetic oil. Thats plenty to clean up everything I've done. My oil looks fresh after 1000+ miles.