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There are lots of tales and myths about antifreeze, not only for small diesels in boats but also for engines in cars and trucks. The push for Dex-Cool seems to have originated from Mack Boring, not Yanmar, but it certainly won't hurt.

The original reason for the creation of Dex-Cool, which is a joint development of GM and Texaco, was to provide long life service in GM diesels used in cars. The basic antifreeze component is exactly the same ethylene glycol used in most coolant products on the market, such as Prestone and Zerex. The difference is in the additives and the color. Dex-Cool uses a blend of organic acids, while the "green stuff" uses a blend of inorganic silicates.

Both work fine when fresh, but the silicates can degrade after a couple of years.

Texaco (Chevron) says that up to 10% of the "green stuff" can be mixed into the Dex-Cool product without harm. Therefore the insistence by some pundits on the web that the cooling system must be scrupulously cleaned before switching to the "orange stuff" is just nonsense. Ordinary flushing is fine if you feel the need to do something.

Yanmar insists on changing the coolant once per year, but otherwise does not seem to specify any particular coolant.

Regards,
Gene Fuller
Yorkshire Rose, C42 #870