 Diesel fuel C-Tain levels. In many states, the C-Tain rating in diesel fuel is 42 to 45. Most diesel engines operate more efficiently with a C-Tain rating of 48 to 50. One treatment of Hot Shot Secret Diesel Extreme will raise your C-Tain seven points, increase fuel economy, and improve cold starts. Due to a cleaner burn, there are less hydrocarbons and particulate matter in your exhaust.