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Q: engine oil leak and smoke coming out of the exaust everytime I start the car. on 2000 BMW 740i

I am using over 10 quarts of oil between oil changes at 3000 miles, and I have smoke coming out of exhaust pipes every time I start the car. It has 148,000 miles on it. I changed to thicker oil and the problem persists. My mechanic says I need to change the valve cover gaskets. Would that help?
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The valve cover gaskets might help with the external oil leak, but there is a high possibility for a bad breather valve (PCV) which is located at the back of the intake manifold. In some case the plastic pipe on the oil separator valve could break also. It's under the left timing cover. I bet the intake manifold is full of oil. You can remove the intake hose from the throttle and open the throttle plate and check with a flashlight.
Those failures could cause a high oil consumption. Unfortunately you might need to get the O2 sensors, catalytic converters and spark plugs also replaced.

Just went through a simular issue. After I changed out the valve cover gaskets on my 99 740i it began to burn smoke when I started it. Upon inspection of the intake manifold interior, it was coated in oil. After further inspection I noticed that one of the recently replaced valve cover gaskets was not seated properly and was letting air be drawn into the engine through the improperly seated gasket. We pulled the valve cover and reseated the gasket and this solved the issue.

Our theory is that the oil seperator uses negative presure on the oil seperater intake manifold valve (located at the rear of the intake manifold) to keep the intake valve closed and which keeps oil from being pulled into the intake manifold. When the gasket was not seated properly it allowed air to be drawn into the valve cover allowing the intake manifold to suck air and oil out of the engine and into the intake manifold. This is the oil that was being burnt. In addition the inside walls were getting coated with oil, when the car would sit after use this oil would drain down and pool into the air intake ports.
The diaphragm ( a big rubber plate) inside the vent valve goes bad (breaks, hardens up and stuck, etc..) all the times. This is a very common problem, the other - less common, but happens - is the plastic line crack open at the oil separator valve, it's under the timing cover.

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