Oil Leak

About 6 months ago, I discovered that my 1999 Corolla was burning oil. I have the oil changed every 3,000 miles pretty consistently, drive about 400-500 miles a week and have at present 169,000 miles. I had the car serviced 2 weeks ago and today it was bone dry. I have been checking regularly each week and for several months I need to put in about 6 quarts of oil between the 3,000 mile oil changes. There is no oil on the ground, no significant leaks visible in the engine, no blue smoke pouring out the back. I have a reliable mechanic who is mystified by the problem. I would be grateful for any clues to solve this mystery.
Thanks for any help or advice you can give!

are you using synthetic? it will burn off and you might not be able to tell.

Well, gosh, this is where I show my ignorance -- I haven't got a clue as to whether the oil is synthetic! I have had my car serviced by the same mechanic for years, and just recently by a new mechanic who is trying to figure out the problem. I would assume the same type of oil has been routinely used over the years, but I could be wrong. If I can be REALLY stupid, how do I tell if a synthetic oil has been used?

Thanks, Loie

If you paid $20-30 for the oil change, it is dino oil. If you paid $40-50 for the oil change, it is synthetic. Really, in general, mechanics use dino oil unless you tell them differently.

I swear, if I didn't have a haynes manual, I'd be no help at all in this forum.

The following are possible problems:
- Oil pan gasket and/or oil pan drain bolt washer leaking
- Oil pressure sending unit leaking
- Valve covers leaking
- Engine oil seals leaking
- oil pump housing leaking

I swear, if I didn't have a haynes manual, I'd be no help at all in this forum.

The following are possible problems:
- Oil pan gasket and/or oil pan drain bolt washer leaking
- Oil pressure sending unit leaking
- Valve covers leaking
- Engine oil seals leaking
- oil pump housing leaking

The Haynes manual is very useful. My wife's 1999 Prizm uses oil as well, my guess is internal consuption with a little leaking. Common with the 1.8L i think. Anyhow, it's not a great deal on our driving routine. 20miles per day [quart a month], but on long trips it's much much worse.