2 reasons for oil level deceasing..
Leaks (Not usually the case as it is very visible at the place where you park car..)
oil burning with fuel... may or may not result in a smoke..
get ur car checked on computerized tuneup..and get the emmission checked... if ur engine is weak.. and is consuming oil.. u need to put in slightly thicker oil..
another thing which you need to know is that if the oil level at the dipstick is at the half..it does not means that the oil in the engine is half... the ratio is not the same for the dipstick...
but you should not ignore if the level is decreasing...
please give us sum detail about your car... so that ppl might advise you whih oil to use...