Speaking of additives Subaru has long recommended adding Subaru Cooling System Conditioner to the cooling system when changing coolant. Subaru began this recommendation two decades ago to address head gasket failures. I have changed the coolant several times on my high-mileage 2010 Forester and have always added the conditioner to the fresh Super Coolant, even when an independent Subaru-only shop recommended no doing so.
After 230k miles my 2010 Forester has never had a cooling system leak or other issue (such as clogged heater core) and it still has the original radiator and head gaskets. To this day it has no evidence of a head gasket leak either externally or internally. I last had an oil analysis done at 190k miles and Blackstone reported 0.0% antifreeze in the oil.
I don't know if Subaru still recommends adding the conditioner on the 2020/2021 Crosstreks but the following recommendation appears in the Warranty & Maintenance booklet for our 2019 Crosstrek: "To prevent cooling system leaks, always add Genuine SUBARU Cooling System Conditioner whenever the coolant is replaced".
^ You do know why mechanics advised against the additive? It's actually a rebranded bottle of "Holts Stop Leak ". Basically a liquid bad-aid goo whose only functionality is the ability to clog little holes. Any symptoms of external HG leak were thus delayed without preventing the damage itself. A perfect band-aid solution. Mind you, if your objective was to postpone a HG change.. it could work, for a while. The drawback and reason why it irked mechanics was that the product also had a tendency to clog the radiator itself. For drivers who neglected regular maintenance, it caused more problems than it solved.
One of the reasons contributing to HG leaks, save for their bad single-layer design, was a change in the pH of the coolant, acceleraring their rot. A long-term preventive maintenance was simply to change the coolant more often that it's recommended, especially in climates with cold winters. That being said, there is no evidence whatsoever the FB20 is prone to HG leaks in the same ways the EJ25x was, so no issues there (yet).
I agree that some stop leak products cause problems but this has not been my experience with Subaru's conditioner. The fact that my 2010 has had no radiator or heater core issues after 230k miles of using the conditioner speaks for itself. I've heard, but do not know factually, that Subaru adds the conditioner to the factory installed coolant.
^ The funny thing is that since you don't have any leaks, there is no way of knowing whether it's because there is no hole, or whether the really small hole has been clogged by the conditionner. Given the mileage, I would actually vouch for the latter, which proves your point😁
Bottom line though - it's fabulous. With 370 000 km on OEM gaskets, you certainly beat the odds, and I hope it continues. Out of curiosity, what did you have to repair/change (beyond usual wear & tear items?)
93k miles - replaced control arm bushings (cracked & worn)
117k - replaced radiator cap (failed to hold pressure at high altitude)
126k - replaced driver's seat cushion frame (weld broke - Subaru covered expense as goodwill gesture)
164k - replaced (3) o-rings on A/C compressor (a/c weak)
199k - replaced valve cover gasket set (blown valve cover gasket at high altitude)
208k - replaced intake manifold gaskets and coolant passage o-ring (no symptoms or noticeable drips, just a very minor coolant leak* discovered during timing belt service)
*so I guess I did have a cooling system leak after all which I had forgotten about when I posted above
208k - replaced front control arms & bushings (cracked & worn)
As you can see, nothing major. However, there was a potential biggie at 177k miles when my 2010 Forester had a very minor front differential fluid leak from the rear of the diff where it mates to the front of the transmission. Subaru dealer said they couldn't replace seal and that the only fix was to replace the transmission to the tune of $4600. No way! So considering options, one of which was to do nothing since it was a very minor leak, I decided to take a chance on a sealer. I added 1.3oz of Blue Devil Oil Stop Leak to the front diff and the leak completely stopped shortly thereafter. As of 3 years later the front diff leak has not returned even after subsequent front diff fluid changes where no additional sealer was added. Lucked out on this one!