Mercury - Oil Tank Cap Assembly DFI OptiMax 75-125HP 36-8M0073266. Overview. Anyone who's spent time with a 75-125HP OptiMax knows the oil tank cap isn't something you think about-until it starts acting up. A cap that doesn't seal right can let air sneak into the system, leave a light mist of oil under the cowl, or trip those random oil alarms that make you second-guess your whole day on the water. This OEM cap assembly replaces everything in one go: the cap, base, spacer, O-ring, tether, and th