Volvo Repair: Low oil pressure light, low oil pressure, 4 quarts

Question
QUESTION: I have a 2004 XC70. All scheduled oil changes have been performed by a Volvo dealer. The last oil change was at 75,000 miles in June 07. I have 90,000 miles on the car and am 7,500 miles over the scheduled 82,500 change. The low oil pressure indicator light comes on faithfully every time I start the car and then goes out. I have never seen the Low OP idiot light come on while driving.  I have never seen any oil spots on my angled driveway or any where I park. There is never any smoke out of the muffler. Yesterday I pulled the car in the level garage to vacuum the interior and thought I would check the oil level knowing I was late on the oil change. To my shock, there was NO oil showing on the dipstick. I added 1 quart of oil, still no visible oil level. I added a 2nd quart, still no visible oil level. I added a 3rd quart, the level was finally showing about 2XX on the dipstick. I added a 4th quart to finally bring the level up to the full mark. Here are the questions: When is the low oil pressure idiot light supposed to come on? Could 4 quarts of oil be used in 15,000 miles?  What damage could be done with only 2 quarts of oil in the crankcase?  Have you ever heard of this happening?
Thank you in advance.

ANSWER: Eldon, you got lucky.  NEVER go over 5000 miles on an oil change.  These cars are known for burning oil.  You should check your oil every other fill-up. Roger

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QUESTION: Gee, that's really an EXPERT answer. Now, would you mind answering the question:When is the low oil pressure idiot light supposed to come on? Could 4 quarts of oil be used in 15,000 miles?  What damage could be done with only 2 quarts of oil in the crankcase?  Have you ever heard of this happening?

Answer
Eldon, To put it mildly, Volvo Engineers decided for some ungodly reason that by the time the low oil light comes on, its too late.  Bearing will be spun and major damage can occur.  You seemed to have caught it in time since you didnt hear any noises happening.  Yes, I have seen it happen and Yes 4 quarts can burn in 15,000 miles. Instead of being sarcastic, change your oil more often and you wont have to worry about running it so low.  Good day.  Roger