A typical service rating also includes a second letter. For instance, if the prefix is CH, the "H" represents the additive package used in this oil. Additive packages include detergents and anti-wear, anti-foam, and dispersant agents; the later this letter occurs in the alphabet, the more recent and sophisticated the package. As new letters and additives are introduced, manufacturers cease offering older ones. Therefore, if your engine manufacturer specified CE or CD oil, you'll have to choose a letter that follows E in the alphabet because these ratings are now obsolete. For example, CI-4 oil is a suitable replacement for CD, CE, CF-4, CG-4 and CH-4. (These prefixes were initiated by CA, which was introduced in the 1950s; CE and earlier oils are considered obsolete.) The number 4, by the way, means that the oil meets 2004 exhaust-emissions requirements, a category typically not relevant for most sail auxiliaries and generators.