[2000SoCalBoxsterS]

Does this happen after you have not driven the car for several day?

Here is what the PCA.org tech section Q&A says about this:

I would not worry about smoke on startup unless you are killing mosquitoes. All of Porsches flat 6 motors have a tendency to smoke occasionally on startup. I would not be concerned about this.

There are several sources of it. One is that moisture condenses in the exhaust system after you park the car for the evening. If it is humid and warm, and then cool overnight, it happens even more. Next morning it will burn off. You will see water droplets coming out of the tailpipe as well. Water is actually one of the combustion byproducts, as it turns out when hydrocarbons are combusted, some of the hydrogen molecules mix with some of the oxygen molecules 2:1 and you get H2o. Most of that will turn to steam since the exhaust system gets hot quickly.

It is also normal for a little bit of oil to be burned off from a cold engine. The pistons are sealed by rings, and the rings are lubricated by oil. But as the piston moves, it does not spray any cleaner to clean off the oil on the part of the cylinder wall it just left. That oil film becomes part of the combustion environment and is partially burned. The next stroke down, the rings bring fresh oil and all is well, except for a bit of smoke.

Now add to this the fact that the engine is horizontally opposed. That means that oil inside the cylinders will slowly collect at the bottom (side) of each cylinder overnight. In turn, some of that can slowly ooze past the rings. Not much, and not fast, but some can. That will burn off at startup.

Lastly, there is the head gasket. It is not feasible to have a 100% perfect head gasket, and still be able to disassemble and reassemble an engine. For example on some of the legendary race cars, Porsche did away with the head gasket and used electron-beam welding to weld the heads to the cylinders. This is frighteningly expensive. Therefore a normal head gasket is used in our Boxsters and 996s. A normal head gasket must seal best at operating temperature. If it leaks a little before warming up, you will see a bit of smoke. As long as the coolant level is correct, and stays that way, or if you can maintain it that way, then there is no problem.