Look at your owners manual. I recall that 5w30 is approved for the chilly temps where you live, but you should double-check.

I'm lucky. I can run straight 40 all year where I live. =)

 

where ya live whats straight 40 and what are the benafits of it?

 

Mobil 1 5W-30 is perfect for winter use unless temps drop below -20 degrees F. There's a chart under the maintenance section of your Owner's Manual.

You didn't mention how cold it gets in your part of Michigan but Subaru recommends a multi-viscosity 5W-30 weight motor oil from sub-zero temperatures up to 100 degrees F. The reason why they prefer a multi-weight oil is for better cold-start characteristics and slightly better fuel economy. Mobil 1 5W-30 is one of those motor oils rated as "Energy Conserving". My fuel economy is pretty good for a WRX but it's probably related more to the way I drive while commuting through traffic.

I use Mobil 1 5W-30 in my WRX all year 'round. Winters are mild, rarely go below the freezing point but summers can easily reach or exceed 100 degrees. I have faith that a decent, off-the-shelf synthetic can still provide protection in hot weather unless I'm tracking the car under those conditions. In that event, I might switch to a different weight of Mobil 1 or other synthetic (or just avoid going to the track because it's too damned hot with the windows rolled down!).

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