I know, I know. Please don't flame me.
I have a 2012 WRX Premium (EJ255). It has about 87,000 miles on it (140,000km). I change the oil religiously at 3,000 miles. I typically run Castrol Edge High Mileage 5w-30 and a blue OEM filter (switching to the black type once my stock of blues run out). I end up adding about a quart between oil changes to keep it right at the full mark. I just sent my first Oil Analysis Sample to Blackstone and am eagerly awaiting the results. My engine was recently rebuilt (new piston, piston rings, head gaskets, exhaust valves, etc.) around 75,000 miles due to excessive blow-by.
Now that the weather is getting warm I've been considering switching to a 5W-40. Everywhere I go online I see people suggesting the 40. I asked my mechanic (owns a Volvo/Subaru specialty shop) and he said that I should always follow the manual. It lists 5W-30 and doesn't have a "or 5W-40" as I understand some of the older models' manuals do.
I don't drive the car very hard. I'll do a pull every once and a while but I rarely run it out to the rev limiter / launch the car, etc. I used to do track days and auto cross with an old BMW E30 and initially intended to do the same thing with my WRX, however... Now that it is my only car, my daily driver, and with just having the engine rebuilt I don't think I'm going to be racing it in any form for a while.
The car does still have some blow-by. Even at idle, if I take the oil cap off while it's running there is a slight "puff, puff, puff" but not nearly as bad as it was before the rebuild. I was talking to a friend and he suggested thicker oil may help that situation slightly, but I'm skeptical. Mainly, I want to be giving the engine as much protection as I can. I want this car to go at least another 80,000 miles and I don't mind spending the extra money to make it happen. I'm also open to any other reliability/long life type mods you guys can come up with.
I still just have a hard time understanding why the manual specifically states 5W-30 but so much of the enthusiast community says 5W-40 is the only thing they'll run. I know the difference isn't huge and I know I will see a slight change in fuel economy (to which I'm totally indifferent) but why wouldn't everyone (including SOA) say that 5W-40 is recommended if it offers the best protection? Just because they want to boost the MPG number slightly? Again, I don't care how much more the oil costs, of if it has any effect on MPG. I just want to know if 5W-30 or 5W-40 is a better fit for my specific car and my specific driving style. Is it not necessary since I have a laid-back driving style? Are there any downsides to running 40? Should I change depending upon temperature? (Southern Ohio - 0ish minimum in the winter, 95ish max in the summer).