My knowledge from being into modular mustang racing is this. 5w20 is good, 5w30 is good, no more than that. You can run a 0w of either of those. In fact having talked to a few mustang engineers, the motor is designed for 5w30 but 5w20 is recommended for emissions reasons. I personally run Amsoil 5w30 in my modular. Never use additives. I finally got my buddy to stop using Lucas and he was like "Wow, it revs faster and doesn't tick".