Over time pigment may separate from oil in the tube. To resolve this situation store your tube upright on the cap to allow gravity to pull the heavier pigment to the cap. This will alleviate oil from seeping out when the cap is opened.
Remember to wipe away paint that may be around the neck of the tube so your tube is easy to open and the cap aligns correctly with the tube threads providing for complete closure to maintain the integrity of the oil paint.
If the cap is stuck, gently use pliers to unscrew the tube or dip the cap into hot water then use a cloth to unscrew the cap.
Be wary of over tightening your caps!