I like to save money and buy my oil in cases. (6 quarts) Obviously I never finish a case in one oil change. Does motor oil go bad over time or will it be fine to use 3 months after purchase?
3-6 month old unused oil should still be better that what's going to come out of your car when its time for a change anyway. I bought a couple cases of mobile 1 too.
"Technically, engine oils have shelf lives of four to five years. However, as years pass, unused engine oils can become obsolete and fail to meet the technical requirements of current engines. In recent years, engine oil specifications have changed every 12 to 24 months. An oil that is a number of years old is simply not formulated to meet the requirements set for newer engines."
Note that no distinction is made between synthetic and mineral oils in terms of shelf life.
