If it's real sciatic pain, nothing you rub on will help. You'll need painkillers and/or anti-inflammatories....and possibly a doctor if it's really bad!
until then, try a bag of frozen peas wrapped in a teatowel or pillowcase and laid on the root of the pain. Remove it once the pain subsides and be careful not to damage your skin. the peas can be refrozen and re used but don't eat them.