My owners manual gives two option depending on the weather. It says you can use a thicker viscosity in hotter temperatures. My old jeep eats up oil and it gets over a hundred degrees where I live, so I’ve been using the higher viscosity with no problem. I read that all a higher viscosity oil will do is decrease fuel consumption by a little. I don’t notice it because Jeep engines have crap mpg’s to begin with.

Yes, stick with the manufacturer’s recommendation. High ambient temps will not affect oil viscosity quite as much as, say, heavy track use. Some people put heavier weight oil in their race cars for this reason.

I would just stick with the factory recommended 5W30. For the most part, oil is oil. Just make sure the oil you buy meets the specifications in your owner’s manual. If you don’t have the owner’s manual, you should be able to download a free copy from Subaru’s website.

Run whatever oil viscosity is recommended in the owner’s manual. You can run a high mileage oil if you are worried about leaks. If you have no oil leaks, just a regular oil matching the manufacturer’s recommended viscosity will do fine.