Engine Oil Vicosity

From time to time we get questions about engine oils and various aspects of how they are rated and what the designations mean. One of the least understood may be the numbers on the front of the container.

In the old days oils would have a single rating such as SAE 30. That rating was simple enough to understand as this was termed “30Weight”.This meant at operating temperatures a “30 Weight” would be thicker than a “20 Weight” and thinner than a “40 Weight”.

Then multi-grade oils came on the scene for good reason. These oils met SAE specifications for the low temperature requirements of lighter oil and the high temperature requirements of heavier oil. You may have also heard of them referred to as all-season and all-weather oils. All of the engines Foretravel currently use are factory filled with a multi-grade SAE 15W-40.

In simple terms, oil is rated for viscosity by heating it to a specified temperature, and then allowing it to flow out of a specifically sized hole. Its viscosity rating is determined by the length of time it takes to flow out of the hole. If it flows quickly, it gets a low rating. If it flows slowly, it gets a high rating.

The temperature requirement set by the Society of Automotive Engineers (SAE) is 0° F for the low temperature and 210° F for the high temperature. Oils meeting the low temperature rating have the “W” (winter rating) after the viscosity (example: 15W). Oils meeting the high temperature requirements have no letter (example: 40). So a 15W-40 would have a “winter rating” of 15 at 0° F and a 40 rating at 210° F.

The wider viscosity range of multi-grade oils allows for much wider temperature ranges than single weight oils. Most modern multi-grade oils are formulated from various oil grades and additives so the oil will have the best characteristics of both thick and thin viscosity oils. Better flow at low temperatures allows for easier starting and better cold start lubrication, yet retains enough thickness and film strength at the high temperature range.

For our diesel engines the SAE 15W-40 is a good all season multi-grade and is good for engine starting down to approximately 10° F and provides for best engine durability at higher ambient temperatures. It will work well for the typical areas of the country you will be heading off to in your Foretravel.