2004 Buick Lesabre uses quart of oil in 3K miles. Is this normal?

Have you had the timing checked. You do not show how many miles are on this vehicle, but the timing belts could have slipped

check your crankcase breather may need cleaned. so you get back pressure.

Usually the problem is piston rings. But it could also be something going on with the PCV system.
It could even be something as simple as changing the brand or weight of oil you are using.
Two quarts of oil in 3000 miles is not a huge amount, so it may be tough to find the cause. One quart with every tank of gas would be alot.

Though I am not familiar with the 2007 model, on the e46s, I know that the low oil light comes on when you are down only about 1/2 quart. Does your model have a dip stick?
A half quart in 3000 miles is not cause for concern. Most of them burn a bit less than that, at least during the first 200k miles, but some burn more. It depends to some extent what kind of oil you are using. It will burn more Mobile1 than it will Castrol Syntec, for example.
I have an '04 330 that uses about a half quart in 7k miles between oil changes, and an '04 325 that goes through a quart every thousand miles. I am only a little concerned about the 325. I would not be at all concerned about a half quart in 3k miles.

Head Gasket

Stop using it

all engines burn oil this is normal

According to GM 1 quart per 1,500 miles is an acceptable oil consumption tolerance. 1 quart in 3k isn't bad.

I've had several gm 3.1 and 3.4 liters that have had there oil consumption issues remedied after the lower intake gaskets where replaced due to coolant leaking.

There are a couple of common problems that can cause oil consumption. Valve seals, bad rings, and PCV valves.

Valves seals would require a valve job which would requirethe removal of the heads. Rings would require a motor rebuild. Both of these are expensive options and oil is still pretty cheap.

Replacing the PCV valve might help. On the 3.8 it will be under the MAP sensor on the passenger side of the intake plenum.

As for as the intake gaskets, its a possibility but I don't have any hard evidence this could be the problem on a 3.8. Eventually the gaskets will start to leak oil. If you decide to replace them at that point I guess you will find out. I wouldn't do it on a hunch, its not a cheap repair.

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That knocking noise could be due to low oil. There could be a leak on the following places - drain plug, on the crankshaft seals, camshaft seals, valve cover gasket, engine may be burning oil (blow by) due to bad piston rings, bad, valve seals. Regularly check your engine oil level and tap off to avoid engine failure.

How many miles do you go before you have to add oil. GM actually states that one quart of oil every 2000 miles is considered normal. I personally think that one quart every 3000 miles is normal esp at higher miles. As an engine wears things such as valve seals and rings get worn and allow oil to seep past and will burn, this can be in such small amounts that you will not notice the smoke and is normal engine wear. The life cycle of an engine is considered 100k miles by the designers, even though we all know they will go far beyond that. If your car has the 3.1 or the 3.4 liter engine you may want to check in the engine coolant for signs of oil, it is a common problem for the oil to seep into the cooling system but is not usually enough to make a noticable drop in oil level. If you are only using about one qt every 3k miles or so i would not worry about it if you have had it checked for leaks already.

10w40 will not hurt the engine but you may still burn a quart of oil every 2000 miles