Many people use lemon oil for polishing and cleaning their guitars. However, I was told recently that "lemon oil" in fact is lemon-flavored petroleum distillates (solvents). If it is true, will lemon oil damage guitar finish on the top/side/back? Is there any other oil that can protect guitar from cracks due to the weather?

if ur goin out to buy a specific product, u might as well just get a guitar polish/cleaner direct from a music store.
most guitar manufacturers make their own mix of oils.
use the brand that matches ur guitar if u want.
get the cloth while ur there too.
BUT!!!
the best thing u can do for ur guitar, as far as preventing cracks, is too keep the humidity around 45%. i cant stress that enuff, especially with acoustics.
get a little humidity meter and a portable guitar humidifier if its really dry at ur place.

Not sure about using it on the body to polish it, but I do know many people use it for the fretboard. Other people use different stuff, I personally use olive oil for the fretboard, I trust it doesn`t damage it since I got this tip from a respected guitar luthier. What I do every string change is remove the old strings and use a brush (Not too stiff). I brush away all that old buildup on the fretboard and then use a paper towel to lightly apply olive oil.

As for polishing and cleaning the body, try to find a silicone free formula, it`s what luthiers use. Dave, is absolutely right about humidity, that`s the best way to prevent weather induced cracking, also avoid long periods of direct sunlight.

If I`d like to use oliver oil to protect the fingerboard, what kind of oliver oil should I use? Is the oliver for food (that one can buy in the supermarket)?

yep just use pure olive oil, no additives.

ha! my wife just brought back some extra virgin from Cicly
- I`m gonna try that!!