How long does it take for gasoline to go stale?

[Krosigrim]

If any oil were to leak out, it would burn off... dark as you say. But there would have been an odor as well. Each fluid has a distinctive smell, especially when boiled. I dont know what you saw, but im offering my experiences as an extreme example.

 

When a car sits, things happen that are not intended. Fluid settles, goes to the bottom. If after a few startups this "dark smoke" occurs, you may have some troubles. Hopefully its just random shit burning off that will pass.

[Shibathedog]

Yeah the smoke went away, it was just some oil burning off. I'm going to take the alternator to be tested and exchange the battery soon.

[ken_cinder]

More likely condensation, I bet there was a bit of a puddle on the ground under the tailpipe. That's another thing you have to watch for when leaving a car sit, condensation accumulating inside the exhaust and working it's way up into the engine. It will get up into any open valves and ruin your seals.