For your own sake, learn to manage your oil yourself. Petrol attendants are not mechanics and every car is different, so they don't know what they are doing or what the best oil for your car is.
Reading an oil dipstick is simple. Read the manual and maybe get a mechanic's opinion on the best oil for your car, and stick with that oil.
Unless your car has a leak, you will not need to top it up more than every few months.
Also low oil on the dipstick does not mean your car is out of oil (or anywhere near being out of oil. The bottom mark is just the lowest recommended level. Usually you want to keep it between the two marks, not necessary maxed-out (more is not better).
If your car was really out of oil, it would move about 10 meters before the engine was destroyed. That's if you ignored the horrible noise and tried to drive it.