If we looked at probability... I would say for defo head gasket.
If the mayo was on the dip or the oil filler cap, then perhaps with short distances yadda yadda....it might not be HGF.......but with it being on your expansion tank cap.. that says oil has got into your cooling system rather then condensation being the culprit with the oil. Short distances etc. would only give mayo, usually on the base of the dip stick, and sometimes around the thread of the filler cap.
While it is inportant to have it checked out with a compression test (Which you can do yourself) If I was a betting man.. I would say without doubt.. Head gasket I'm afraid. Until you have it sorted out, it would be unwise to drive it or risk further....more expensive damage.