My 2014 Forester specs calls for 0W-20 synthetic oil but I suspect my dealer is using conventional 5W-30 oil. So far I had 3 oil changes and everytime the service advisor mentions 5W-30 but quickly corrects himself when I question it. The sticker inside for next oil change always uses 5000 miles interval instead of 7500 miles. How can I tell if I got the right oil? Would the use of 5W30 cause any harm to the engine? Would it affect MPG?