Hi Friends! I am trying to understand why would a Synthetic Engine Oil maker NOT come out and say openly & clearly that they believe their oil can extend oil change interval upto so & so many kilometers or at least publish TAN & TBN info in the product data sheet? Amsoil says up to 25 to 35K Mobil 1 EP says upto 15K Heard RP about 9k All list out conditions in which OIC can be extended with their respective product. ASSUMING A GENERAL/CONVENTIONAL PASSENGER CAR ENGINE! Here is my question what is the single most important thing in the Oil which really allows it to be used for extended service purposes? ADD PACK or BASE OIL? I have heard conflicting info...some say no if it is made of group IV and above base oil it can be used for xtended purposes and at the same time have also heard that the stronger (higher dose) the ADDITIVES PACKAGE the longer once OCI will be regardless of the base being a highly refined group III. OR The Oil being THICKER has anything to do with being able to be used for xtended OCI's? Please Share Your Thoughts! since I am at it...would Red Line OW-20 can be assumed to meet GF-4!