Subies... thick or thin? (2007 Impreza 2.5i SE)

[Dominic]

Hi everyone,
Once again I'm debating oils out in my head. As most of you know I am a valvoline nut, but I am considering other oils this time. My main concern is engine wear, and second concern is fuel economy. I would like to balance both out as best as possible (I'm sure that's how most people think too).

I am currently running Valvoline Synpower 5W-30 (10.7 cSt)
I am considering the following oils

Valvoline Durablend 5W-30 (10.3 cSt)
Valvoline Maxlife 5W-30 (11.3 cSt)
Valvoline Synthetic Maxlife 5W30 (11.8cSt)
Castrol Syntec Blend 5W-30 (11.0 cSt)
Mobil 1 5W-30 (11.3cSt)

It's interesting to me the M1 is the same viscosity as the Maxlife synthetic, however the M1 meets "energy conserving" requirements somehow?

So... do subies like thin 30wts, or thick 30wts? Also keep in mind I'm not running a turbo and for the first few oil changes I will be going 3000mi with UOAs to get a normal wear pattern going.

 

[Titan]

My opinion is that you won't be able to tell a discernable difference between those oils listed. If you have lots of short trips, an oil that is thinnest at your starting temperature (which may or may not be a 5W-XX, btw!), might show a bump up in MPG. My Subarus just don't show a difference between mpg when changed with oils in the range you posted....one is a small-town mostly around town driver, the other is a highway cruiser. I happen to use PP 5-30 because I think it is a good value when caught on sale, and I use 7500 OCI's. There's a fellow using XX-20 weights and posting UOA's on his Subaru...good wear numbers...that oil might show mpg improvements, if he's a short-tripper.

 

[Sparkman]

Our EJ25 is quieter and uses less oil if I use something relatively thick. I just put some fresh RTS in last night, I use GC in the winter.

If it isn't using any oil and it hasn't started piston slapping yet I don't think that it matters much which of those you use.

 

[Steve S]

I have a hard time with a thick or thin 30 wt .Other than placebo is there any proof that there is a noticeable difference in the number spread. We are looking at 1 cst difference. Gas mileage difference?

 

[Thatwouldbegreat]

Most Subies like 5W-30, and 5W-40, 10W-40, and 20W-50 for higher temps and loads.

I think 5W-30 is fine, and thicker 0w-30 and 5W-30 oils are probably better.

I run GC 0W-30, Elf Excellium 0W-30, and Redline 5W-30 or 5W-40.

 

[Dominic]

That's good to know. I won't feel bad running maxlife synthetic now since it's a little on the thicker side of a 30wt. I guess I kept thinking it was kind of like a Honda engine and might need nearly a 20wt to perform well (However I'm getting 25-28mpg with it so I'm not complaining!)

 

[Titan]

25-28 mpg? I guess you're not on the highway, then. My Impreza-based manual transmission wagon (Saab 9-2X) would often get in the upper 30's on the HWY. My current automatics (Legacy and Forester) get from the high 20's to 34 mpg HWY. 100% small town driving of the Forester just turned in 24...not bad, considering all the idling my wife does in the pre-school "car-line" to pick up a child.

I'd not be surprised if there were more and more people that choose to run 20 weights in these engines...like that one fellow did over in the UOA section, with good wear numbers. That strategy would probably be perfect for the Forester. On my HWY car, which sometimes sees some fairly high speeds for long periods of time, I'll stick with a 30 weight, unless Subaru comes along and back-spec's them to a 20 weight. If they do that, I'd jump to a 20 weight immediately.

I just don't think there's much to worry about...a thick 20 or a thin 30 are so close together...it's not like a thick 20 is going to damage the engine. If it were that sensitive, they'd recommend 40 weights.

I wouldn't be surprised if the 30 weight recommendation (instead of a 20 weight) is related to the boxer engine. With my old boxer BMW motorcycles, if you put the bike on a side-stand...the "downhill" cylinder would smoke on startup, as the oil on that side would slowly leak past the rings when it cooled. Perhaps using a thicker oil helps to keep this leak-by down? Just a guess....I'm not saying I know it for a fact.

 

[Dominic]

The 25-28mpg is around town. In ultra urban (2-5 mi runs, all stop and go... Philadelphia. I see 22mpg (perfect EPA). It's incredible. I have no doubt in my mind I will see 34+mpg on pure highway with it with cruise control! I'm so happy

I think I'll keep running synpower for now and do a UOA at 10000 to verify wear. Thanks Titan for the input. I really am enjoying the car, already got 1200mi on it and I've had it just 3 weeks

My last camaro EPA stickered 19/29 and got 15mpg in town (ultra-urban driving). To actually get the EPA 22mpg in ultra-urban conditions has been incredible. Most people won't ever see fuel economies so low because of where they live.

 

[TeeDub]

The top tier, PAO/Ester SAE 0w-30 synthetics are so good, that I'd just go that route and not even bother with 5w-30 or 10w-30.

I experiment with all sorts of different grades in my personal vehicles (I have the Amsoil 10w-30 Marine oil in my Outback at present), and I've seen NO advantage to thicker oils in engines that aren't excessively worn. This is true even in my turbocharged Audi, when comparing 5w-40/10w-40 to 0w-30. Wear rates and oil consumption are essentially the same, but the 0w-30 provides the best engine performance and highest fuel efficiency and the turbo seems to "spool" up quicker.

As an aside, you'd really have to go up and down at least 10% in terms of high temp, high shear viscosity to notice any significant differences in engine performance. These oils listed are all 30wt grades with HT/HS viscosities within +/- 5% of each other. Any differences here would strictly be some placebo effect and not one related to fluid viscosity considerations....

TS

 

[SubLGT]

I was surprised to learn that Subaru uses 0W-20 as factory fill for the H4-SOHC (non VVL) motor. All other motors use 5W-30 as factory fill.

 

[DC84]

Many Subaru owners would recommend a Xw-40 weight oil if it was boosted. I use Castrol 5w-40 in my 06 WRX and it seems to be working good so far. Many others like GC but at some point that may no longer be a option for some. As for economy im usually in the low 20's per gallon with a good bit of hard driving. As far as wear goes you will get mixed opinions on whats the best way to break in a boxer engine but for what its worth subaru has told many owners that after the first 1K miles its fair game to make the switch to synthetic. If you look over the UOA section of this site you will see that many subaru's have excellent reports on either dino or synthetic as long as the intervals are within reason. Enjoy the new subbie and make sure to check out www.nasioc.com

 

[Dominic]

Yep Im on NASIOC - Eyedoc Orlando there
Thanks for the help. I'm gonna stick with 5W30 in my nonturbo 2.5i. With gas prices as high as they are, it sure is nice saving 10% on fuel cost just in price/gal alone on my daily driver.

 

[Losiho]

I was previously using Mobil Delvac 1 5W40 in my Outback (EJ25) with a cSt of 14.7 - now using Motul EcoEnergy 5W30 (cSt 9.9) and there is zero difference in engine noise. However, the engine does feel more responsive everywhere in the rev range due to the lighter oil.

 

[tenderloin]

My daughter has an 07 Impreza 5 speed SE The boxer is not the quietest engine on the street. Changed out factory fill at 3,000 miles. Running Havoline synthetic 5W-30 with factory filter