ok this is coming directly from a manager of a bmw dealership. if you have an e36 and have been running on regular oil DO NOT switch to synthetic as it will ruin you r engine. the best thing to do if you insist on synthetic is to place 5 quarts of regular and 1 quart of synthetic. oh yeah in regard to 89 vs 92 octane..92+ octane will only make a difference if your driving a high performance vehicle like a ferrari or m5.