Is it safe to use magnesium oil if I have low blood pressure, there is a warning on the bottle that says not to?

How low? If you have BP that's low, but is normal for you, it shouldn't be a problem. If you have sudden drop in BP that might have damaged your kidney function, potassium and magnesium should be used with medical guidance or avoided. Intravenous magnesium lowers BP but taken orally, it doesn't. Too much can cause diarrhea.