we're not talking hoat we're talking red (oat) or green ( traditional ethyl glycol)
lets not just limit our options to just ethyl glycol or oat If we really want to make sure we cover all the possible options that he could put in the engine.
HOAT
soluable oil
light machine oils
water
sea water
diesel
hydracaunoil (or however its spelt and pronounced)
skydrol
IMHO HOAT was introduced because its compatible with both OAT and the traditional green stuff which makes it less of a headache to use and is whole lot less maintenance intensive if you mix it with either of the two typically incompatible coolants.
What I use in an engine is as simple as looking at what the manual says and the manual sayeth OAT. If using the manufacturers spec item causes a problem and using something else masks it then your not actually fixing the problem you're just hiding it until it comes back bigger and uglier to bite you in the tail at a later point.
Short cutting correct maintenance might save you a little money in the short term but its going to cost you or someone else down the line in the long run.

