Yes. Every vehicle (to my knowledge) has one, other wise how would you be getting a reading on the dash (or what would trip the idiot light if you dont have full guages).
How low are we talking? Factory spec says anything down to as low as I believe 12 or 13psi HOT is OK. Personally, mine doesn't dip below 19-20, even after a long run down the highway in 100* temps in the summer.
The oil pressure will fluctuate, and even more so once the oil has been warmed up for a while (say after a long drive).