"I purchased some oil 2 years ago. Firstly, is it safe to use? Secondly, does it lose potency over time?"
Light, heat, air, and time all effect the shelf life of cannabis oil. Unfortunately, there’s not really a standarized shelf life time frame. As time passes, the chemical properties of all concentrates will degrade. You can control a few elements. It is best to store your product in cool, dry, and dark places. It’s a good idea to properly store cannabis oil to increase the unknown shelf life. As for your product that is 2 years old, I do not recommend using if you don’t know what elements may have affected it.
I agree with the previous respondent. Also, just like your kitchen oils and body products, cannabis oil can and will become rancid if not stored perfectly, and just like those items, it must be used within a fairly short period of time. I have switched products and now and alcohol-based tinctures which don’t have the troublesome shelf life issues, or if I use salves, I make sure I use them within 3 months and keep them refrigerated at all times.