What he said -- ^^^^^
Don't overthink this. Change ALL fluids regularly - period determined by the system. Do that and you'll be light years ahead on the big bits functioning as they should.
Any 10w-30 ought to work just fine. If there's a particular brand you like, go for it. On a 300k mile engine, I wouldn't spend the money on synthetic - I'd just shorten up the interval a bit and use regular dino oil. If you're gonna go synth and use a longer interval, depending on what the interval is, you might want to consider a filter change in the middle of that longer interval.
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82 242-6.2L; '17 Mazda3; '16 Crosstrek
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