What do the numbers and letters in a motor oil designation mean? Good question. Here's the answer.
First, there's a two-letter code indicating the type of detergent package that the manufacturer uses in the oil; this looks like SE, SF, CD or such. The S codes are for gasoline engine applications; the C codes are for diesel engine applications. The second letter is assigned in sequence as new levels of protection are developed; thus SF is considered better than SE, SE is considered better than SD, and so forth.
The more noticeable designation is the oil weight. This is either a single number (e.g., 30 weight) or a pair of numbers separated by the letter W (e.g., 10W30.) The latter type is much more commonly used these days, and are the only type that most automobile manufacturers specify in operators manuals. The first number in the designation (10W) is the apparent viscosity of the oil when it is cold; the W stands for `winter'. The second number (30) is the viscosity of the oil when hot. There is a trick here; the oil doesn't actually get thicker (turn from 10 weight to 30 weight) as it gets hotter. What is actually happening is that when the oil is cold, it has the viscosity of a cold 10 weight oil. as it gets hotter, it doesn't get thin as fast as a 10W oil would; by the time it is up to temperature, it has the viscosity of a hot 30 weight oil.
Note that these numbers actually specify ranges of viscosities; not all 10W oils have exactly the same viscosity when cold, and not all 30 weight oils have the same viscosity when hot. Also the behavior of multi-grade oils is caused by additives, and it has been reported that some 10W40 oils do not retain their multi-grade characteristics well over time. But 10W30, 15W40, and 20W50 oils work fine.
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