Hello, I typically have my oil replaced with conventional (non synthetic) 10w30 in non winter seasons in my 2004 WRX wagon. I live in coastal NH.
My recommends 10w30 from -5F and up although conventional is only spec’ed. They do spec 5w30 below 0 and up.
In the winter I like to use synthetic engine oil for easy starts on those rare times it gets below 0 and less engine clatter (it a Subaru 
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Is 10w30 Synthetic engine oil more flow able below 0F vs say 5w30 conventional?
If I owned a vehicle with a turbocharged engine I would use nothing but synthetic.
A 5w oil will flow more easily at low temperatures than a 10w oil, regardless of oil type. I think I’d use 5w30 synthetic as a winter oil in this car.
Why not use 5W-30 Synthetic. That’s what I’ve been using for the past 15+ years.
I don’t think the synthetic oil will flow better then the conventional oil at the lower temps…but it does provide better protection in that with synthetic oil you don’t need as many additives to make it a multi-weight oil. The synthetic will be a purer oil, thus the better protection…especially at startup.
Is 10w30 Synthetic engine oil more flow able below 0F vs say 5w30 conventional?
No. The specs are the 10W and the 5W The synthetic will flow the same as any other synthetic or conventional 10W oil at 0? Conventional oil will flow the same as any other conventional or synthetic 5W oil at 0?
Of the two you list the conventional will have better flow when cold. They will both have similar flow when hot.
Personally I would re-check the specification in the owner’s manual and I would likely use 10x30 synthetic in your situation if it meets the manufactures specifications.
I agree. The turbo puts alot of stress on the internals, using synthetic is cheap insurance. Just use a 5W-30 synthetic or if you’re neurotic enough one of the really lightwight 0W-20 oils.
Subaru owners and multiple mechanics report more consumption with 5w30 synthetic vs conventional 10w30. No idea why.
I will state I am changing the oil every 3.5-4 months which turns out to be every 1500-2500 miles.
Over the operating range indicated by OP, the difference in viscosity between a synthetic and non-synthetic of the same weight is extremely small. However, outside the range, say -40F or very high temperatures, the synthetic will have a lower “pour point” and and higher “flash point” than non-synthetic. In other words, a synthetic is more stable; that’s why it is recommnded for very cold andf very hot (including turbo) applications.
Why? because the 10w oil is thicker and the piston rings are worn to the 10 viscosity thickness.
At 0F, any synthetic oil will flow better than dino oil of the same weight, and often better than thinner dino oil to boot. Have you ever poured oil on a cold winter day? Get yourself a quart of pure synthetic (can’t go wrong with Mobil1) and a quart of conventional oil, leave them outside overnight and then try to pour them. It’ll be obvious who the winner is. I know my old car is much happier in the winter with synthetic.
Unless the car goes to the top of MT Washington in the middle of winter it’ll NEVER see anything below -20. And as for very high temps…Again…not likely here in NH.
At 0F, any synthetic oil will flow better than dino oil of the same weight…
If this is true, then what does this say about the validity of the API weight standards, like 5W30, which specify the pour point characteristics at 0 degrees C and at 100 degrees C?
Are you saying there are two different API standards in use - one for dyno oils and one for synthetic oils?