I've been using Mobil 1 in my cars since the late '70s; I still have a copy of April 1976 Popular Science that addresses synthetic oil. I had a '76 Rabbit that I had used Mobil 1 in since 2000 mi. I changed its oil every 100,000 miles and added a quart every 27,000 miles. It had the head gasket fail at 198,000 miles, and the hone marks were still in the cylinder and there was NO ridge at the top of the cylinder; nein, nichts, nada. My question for those with some expertise with oils is if I mixed Mobil 1 on a 1-for-2 or 1-for-3 ratio with my Aeroshell 15-50 semi-synthetic, would that create a problem? Opinions, please!