@GeorgeSanJose from what I understand, Subaru had head gasket issues because of the material they used to use from about 99-02. They have since redesigned the gasket. It should be in good shape, after you replace it.
It is more prevalent on their 2.5 and 2.2 liter engines. DOHC especially are more suspect.
The SOHC seems to have a lesser number of incidents than the DOHC.
The 1.8 engine tends to not have head gasket issues.
@DaveJ, the dreaded head gasket issues with Subarus usually leak coolant or have a breach so that coolant gets in the oil passages or cylinder head. That’s usually a really bad problem: The car usually overheats at best or totally ruins the engine at worst. When it overheats, the heads usually warp and need to be remachined.
In your case, since the car has not overheated and is just leaking oil, you may be able to get away with just changing the head gaskets without having to have the heads machined. Remachining the heads usually does buy you longer life because they usually also do other good things to like like the valve seats, etc - it depends on what you want out of that car. Some people snug the head bolts down in an effort to stop the leak. It may work for you/it may not. I’d replace it to make sure.
I’d still get the timing belt, water pump, thermostat, seals, idler, etc replaced because they are there anyway. The parts cost is relatively low and labor is negligible, when they have that thing half apart anyway.
I would do it soon, before the rest of the gasket fails.