Based on my experience, there are zero negligible effects when switching between synth and dino oils. Not sure where that all came from but I've found it to largely be a bunch of malarkey.
In my ice racing days I took an engine that was raised on synth oil and swapped back and forth a few times between synth and dino oil. Nothing blew up or broke as a result.
When I started racing motorcycles, my first race bike had synth in it. Again, I used dino oil and nothing happened.
Couple of turbo cars sprinkled in there over the years, all of them went from synth to dino oil. Nothing happened.
To simplify things, I have been purchasing 5gal buckets of Traveller oil from TSC for over 10 years. That oil has gone into everything I've owned with an engine...the ice racer, the race bikes, my truck, couple of Subarus, the tractor, wood splitter, chipper, push mower, etc.
If I were to follow every mfg's oil suggestion, I'd have 11 kinds of oil to keep track of. Yeah, no.
I too have had a whole lot of oil samples tested by Blackstone Labs over the years. There's a lot to be said about the results of spectral analysis of oil samples.
If I were OP, I'd bump up the oil weight to a 10W in that Subaru and just continue using dino oil.
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