I have been using Mobil 1 synthetic in my 89. The reason for using this is because the previous owner told me that was all he used. I have heard that once you start using a full synthetic you can't go back. Is this true? I want to change back because I don't drive the car much but always change the oil after 3 months. The big money spent on the Mobil 1 seems unnecessary. My car has 99k on it and I have heard that it is better to use regular oil because the synthetic can cause problems with particles breaking loose from seals.