I am a new owner of an 81 308 GTSi. My oil pressure light went on and I only had standard 20/50 oil in my garage. the car was low 3 quarts. About two weeks later I was driving down the freeway and then my "slow down 1-4 cyl" light went on. I was told that mixing standard oil and synthetic may have fouled my plugs and that is what is setting my light off. I plan on changeing my oil and plugs. if this doesn't work. what could be the problem? PS the car feels it lost some of it's bottom end