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Can switching motor oil viscosity harm my engine?

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Question: The boys at the Ko Olina harbor are having a heated discussion concerning motor oil. Since Costco hit town, we’ve all been able to buy Chevron 15W-40 Delo 400 heavy-duty motor oil at ridiculously low prices. But what about the older engines we’ve been using straight 40W Delo 400 and other single-viscosity products in for years? Can we damage our dependable old motors with this stuff?

—Armand Charron Ko Olina Resort, Hawaii

Professor Diesel: Before you switch to a different brand or type of oil, check your engine’s manual to see what the manufacturer specifies. The manufacturer’s tested to see which oil protects your engine best. Don’t second-guess the experts!

There are two basic specifications you need to be concerned with, essentially. They are promulgated by the Society of Automotive Engineers (SAE) and the American Petroleum Institute (API) and are usually listed either in print or in a doughnut insignia on the container that you buy your oil in.

The SAE sets standards for a product’s flowability, typically noted as SAE 30, 10W-40, 15W-40, etc. A multi-viscosity oil like 15W-40 (the “W” means winter) offers more flowability when the engine is cold and less when it’s warm. Straight 40W Delo 400 is different with a single level of flowability.

The API rating indicates which additives (friction-reducing anti-foaming agents, temperature stabilizers, etc.) have been used. There are two basic API classifications: S and C. The latter usually applies to gasoline engines and the former to heavy-duty gas and most diesel engines.

The bottom line? Manufacturers are sensibly strict about SAE and API specifications. Extensive harm can come to an engine for a variety of reasons, not least of them being the use of the wrong oil product.

Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.


This article originally appeared in the September 2011 issue of Power & Motoryacht magazine.