Both are common issues with Barrs. It plugs up heater cores because the small passages in there can't handle that amount of stopper. Sometimes it plugs up the radiator too.
As for the coolant light, most of those sensors are resistance based. The coolant gets low, the conductive float sinks, closes the circuit. Well Barrs has a lot of metal in it and it closes the circuit.
You need to get that garbage out of the cooling system. Last time I dealt with that was on a Caddi DTS. I drained and refilled that cooling system three times before the light went out.
GM has these organic fiber-type stop leak tabs. They aren't conductive, I've never had issues with them plugging heater cores, and they work wonderfully. It's been a few years since I needed any, but last time I bought them they were only like $5 a pack.
On some of the Caddi engines you have to use them when you do head gaskets or they will pour coolant out the sides.