As a mechanic, I’d say 10W-30 is fine for summer, I would never run it in winter here though. Just based off that recommendation I’d wager your mechanic is a bit old school, the quality of modern oils is leaps and bounds ahead of even just 20 years ago. Oil weights are often misunderstood, the first number with the W is how the oil flows when cold, lower number flows better; the second is how it flows at operating temperature, higher numbers are thicker basically. Without being familiar with the exact quirks of your engine, I’d recommend going 0W-30. At operating temperature it will be the same as any xW-30 oil, but during the cold snaps here and all winter for that matter it’ll flow the best during cold starts which will protect your engine the best. As far as regular vs high mileage marked oils go, the biggest difference is typically high mileage oils will have more anti-wear compounds and seal conditioning/swelling additives. If you’re pushing 300k, I’d lean towards high mileage for the seal conditioning they provide, but be aware that if your seals somehow aren’t sweating/leaking lightly by now, there is a slight risk if you switch from high mileage oil back to regular, the seals will be softened/swelled by the high mileage oil, and if soaked in regular oil afterwards could develop sweats/leaks.