Heat alarm, guessing solid alarm rite. If that's the case and motor is truly hot. I wouldn't recommend running at wot. When u changed impeller , did u look at ss impeller cup in housing ? Had yammy 70 hp at shop in April customer had replaced impeller and still getting alarm, what I found is stainless steel cup had spun in housing, driveshaft hole in cup is off set just a little and if cup spins just a little now u have an egg shaped hole in cup and loses water from impeller housing up next to driveshaft. And another thing, when lower unit is removed , pull thermostat and take water hose stick in where thermostat sets and back flush. Have someone to hold Gatorade bottle at water tube that comes down to lower unit and catch what ever comes out ( impeller pieces, sand or weeds). As far as oil I don't know, it doesn't get that cold here.