[subiesailor]

First make sure the valve covers seals are not leaking. 2003 its about that time the valve covers will start leaking oil. Under load ie going up a hill this would match your description.

Coolant you need to pop the cap off when the car is cool and make sure it has coolant in the radiator.

HG issues more often than not are tied to a few things with cars beyond the 2002 gasket design problem. If your car sat parked for long periods of time and you have a very spotty oil change history that will actually cause gasket issues. This would fit the valve cover gasket leak ie oil problem you describe and could contribute to a failed HG involving coolant loss. Just about any brand car will have similar issues when the oil is not maintained and the car is stored for a long period of time etc.

 

[subiesailor]

The heater in all cars uses coolant if the heater is not putting out hot air and should be - that means you do not have enough coolant in the system to provide heat via the cabin heater. This means Pull over NOW and put some coolant in the car before you basically destroy the engine.

 

[subiesailor]

Valve cover gasket replacement should not run you more than $300 at a local LEO shop. Be sure they use the subaru gaskets - a few folks have tried cheaper non brand gaskets and had issues with them not fitting or leaking soon after doing them.

Easy check look at the spark plugs if you see oil in the spark plug hole or on the wires near the engine - this is the classic valve cover gasket leaking. Not a big deal but needs to be done given oil will also foul up the wires and cause shorts with the spark plug and the engine will run rough.

 

[subiesailor]

Yes only use a stock Subaru thermostat the cheap junk you buy at the local parts store will cause your subaru to over heat then you for sure might have a blown HG. Very well known issue with non OEM t-stats with subarus. Only use the thermostat you get from the parts dept at the dealer. Same goes for the cap if you replace that