Lots of oil on spark plug means you got a valve seal problem. The only
problem is not alot of people are willing to do the valve seal job...
majority of them wants you to do the valve job which will cost you dearly...
about $1200 I think.
Are you the original owner or your parent? How long has the car been running
synthetic? If recently, then I suggest you go back to regular oil. However,
if oil still good... then use regular oil to top up.
If you can't afford to fix anything now, try some valve conditioner sold at
autoparts stores that supposedly swell the seal to stop oil burning... if it
doesn't work, then you are out of luck. Do not overfill oil level.