If you have stored the motor oil in a cool, dark place and it has not passed the expiry date, you could use it. However, you must check it carefully for its consistency. If it is thick and sticky, and has solid or gel-like particles, do not use it. If you see that the additives seem to have separated from the oil, it is not safe to use. You must also check its color. Usable motor oils have a clear appearance and if you notice any milkiness or haziness, that is a sign of water contamination. A change in the original color of the motor oil could also indicate its instability.
The logical conclusion is that airtight containers stored in the refrigerator would be the best way to store your herb long-term. However, your average home grower usually won’t have access to large refrigerator space. Keeping big containers of weed in your kitchen fridge isn’t exactly ideal, especially since the weed will be exposed to the light every time you open it to get something to eat. For this reason, if you need to store large amounts of your herb long-term, it might be best to do so in the back of a cool, dark, low-humidity closet or area that isn’t highly trafficked.