I am alarmed! You said that it was overheating badly. Shouldn't that recieve attention before anything else? If it is in fact overheating, driving it more is damaging the engine. It's probably someting simple like the thermal switch that signals the system to turn on the fan, or the thermostat, or the fan relay.

If it indicates overheat, but is not overheating, examine the sensor for the temperature gauge.

10/40 won't do a thing to address your engine temp issue. Use whatever viscosity the manufacturer recommends. Correct the problem as jemdandy suggests. Got a '93 Tempo that would overheat sitting idling in traffic, drop to normal driving down the road. In my case the electric fan motor quit.