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Using synthetic oils in older engines

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Question: I own a 1996 43-foot Hatteras with twin six-cylinder Detroit Diesel engines. I change the oil approximately every 100 hours of operation. At this point, I have 1,200 hours on each diesel and so far have only used non-synthetic oil. Is a synthetic or a blend of synthetic and regular oil contraindicated for this aged engine for any reason? People successfully use blends and/or synthetics in their cars. But everyone steers me away from synthetics because my engines are too old. What do you think?

—Carl Smedberg
Indian Harbour Beach, FL

Professor Diesel: I’m a firm believer in synthetic oils. They help engines run smoother, quieter, cooler, and often with less smoke. Boat owners typically choose them to extend oil-change intervals. And this is a worthwhile goal but only if your engine’s manufacturer agrees. These days most manufacturers don’t agree unfortunately, at least in regards to older diesels like yours.

That said, some engine companies do allow the use of synthetics in older versions of their products as long as they meet the same American Petroleum Institute (API) and Society of Automotive Engineers (SAE) specifications as regular oils. A manufacturer might specify an API CF-4 rating (typical for high-speed, four-stroke diesels), for example, or perhaps an SAE 10-40 rating, either one being a fairly mainstream spec.

Manufacturers of today’s high-tech powerplants often require synthetic lubricants in their new engines, in part to meet emissions standards. Such engines are designed precisely for the type of oils specified. I strongly recommend that owners of these engines don’t second-guess the manufacturers and follow their recommendations.

With your engines, I would be cautious converting without first contacting your local Detroit Diesel distributor. Proceed with Detroit’s approval only.

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


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