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.