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Recommend 50/50 glycol based with rust inhibitors.
American Diesel says plain old green ethylene glycol Prestone or equivalent is fine.

Many Lehman users will agree with thousands of hours without issue.

Unlike oil that has changed and improved lubrication...the basics of antifreeze for the Leman remain good enough from years past.

You can use more expensive stuff..often like oil though it is nearly impossible to prove it is really doing your engine any better than the old stuff.
When we bought the boat we have in the PNW back in 1998 we were advised by our diesel shop, acquaintances in the marine propulsion and generator manufacturing industry, and American Diesel's Bob Smith (who has apparently since changed his tune) to use a coolant specified for diesel engines in the FL120. In our case, the most prevalent brand used in this area is Cat. I have no idea who actually makes it for them but it meets whatever specs they set.

In a Jurassic relic like the FL120 you could probably use Skippy's peanut butter and the engine wouldn't care much. So plain-Jane automotive coolant is undoubtedly fine. But since the properly specced stuff is pretty much the same price in the overall scheme of things, we prefer to use what the folks in the marine diesel industry have recommended to us.
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Even the cheapo green Prestone meets these standards...

ASTM D4985 standard for a heavy duty diesel antifreeze*. It also meets ASTM D3306 standard for a high quality antifreeze coolant. It will not affect engine or vehicle finishes, and is compatible with all major American brands of antifreeze and supplemental additives. It will not affect radiator hoses and contains a high quality defoamer to prevent foam for the life of the coolant.

* As per the Cummins heavy duty truck antifreeze coolant specification.

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