Does the conversion of crude oil to greenhouse gases have any measurable effect on earth's gravitational pull? Oil underground is much denser than greenhouse gas in the atmosphere.
Does the conversion in anyway effect the gravitational force from earth.
 A: As Hoytman points out, fossil fuel combustion does not change the mass of the earth, and so its gravitation with celestial bodies is unchanged.  But moving mass from the subsurface to the atmosphere would slightly decrease surface gravity.
Let's see whether that decrease would be measurable.  Let's suppose that, in some dystopian future, fossil fuel combustion has removed so much material from underground that the surface collapses and the radius of the entire earth $R_\oplus \approx 6\times10^6$ m shrinks by a meter.  The volume beforehand was
$$
V = \frac{4\pi}3 R_\oplus^3
$$
and the change in the volume is
$$
dV = 4\pi R_\oplus^2 dr
$$
We'll pretend that the earth was a uniformly dense sphere before and after.  In that case the fractional change in local gravity $dg/g$ is equal to $dV/V = 3dr/R_\oplus \approx \frac12\times10^{-6}$.  This is about ten times smaller than the uncertainties on $G$ and $M_\oplus$, so it's safe to conclude that even in this extreme case the gravitational shift you're asking about wouldn't be measureable.
A fastidious reader might note that if the earth actually shrank as the mass moved to the atmosphere, the effect would be even smaller: also changing the denominator of $g=GM_\oplus/R_\oplus^2$ reduces the size of the effect to $1\times dr/R_\oplus$.
A: Because gravity is a property of mass and mass is neither created or destroyed when fuel is burned, the only measurable difference would be caused by the change in the location of the mass.  When fuel is burned, mass is taken from the surface of the planet and added to the atmosphere.  this would cause a (slight) reduction in the downward gravity felt by objects on the planets surface.  As you move away from the surface of the planet (both up and down) the difference would shrink.
A: This is a pretty academic exercise. The calculation ignores the many side effects to the conversion. Depending on the composition of the oil, you will need to consider atmospheric pollution leading to a greenhouse effect. The atmosphere would contain more water vapour which impacts on atmospheric pressure. While total mass of earth plus atmosphere is the same, surface gravity as well as surface pressure will change. The change in surface pressure would be more noticeable than the change in surface gravity.
