additives don't work with this engine at all.
Swap in an EJ22. Inexpensive and the best engine Subaru has ever made rather than the current engine in your car which is the worst engine SUbaru has ever made. It's not bad, but it's certainly not like they're other platforms in terms of cost and reliability.
1995 EJ22 from an automatic (legacy or impreza, doesn't matter)
1996-1998 EJ22 with EGR and get the exhaust manifold with it.
Car-Part.com--Used Auto Parts Market
Do one of those two options above, install a timing belt kit, adjsut the valves, new valve cover gaskets, spark plugs, plug wires, and cam seals, crank seal, water pump, and you've got another 120,000 miles coming your way.
Buy one for $400 and sell your EJ25 for the same, the engine will be essentially free. They can easily be had for $250 - $600 (high end in subaru rich areas).
EJ22's never fail so there's zero demand and they're cheap.
EJ25's fail all the time so they're expensive and hard to find or questionable used.
EJ25's aren't a good fit for most people - they're so prone to failure that used ones are risky and new ones are expensive and not very palatable if they fail.
Example - keeping in mind i've done this multiple times and helped others:
$400 EJ22 - sell the EJ25 for $400
$200 for timing kit and seals, valve cover gasket
$500 to install.
you just bought Subaru's best engine they ever made, as close as you can get to a reliable 120,000 miles for $700. that's a steal.
EJ25 transactions never approach that kind of repeatability.