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My wife's 2013 Subaru Impreza was due for an oil change. I ended up putting the wrong weight oil in, 5w-20 instead of 0w-20.

What kind of impact will this have? Obviously once it gets to operating temperature, the viscosity is correct. Is 0 versus 5 when cold worth worrying about?

The manual requires 0w-20 and does not provide for any alternative weights.

I'm in southern PA, so temperatures are between 30 and 40F in the mornings.

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  • Why do you think random internet users will give you the nod to use an oil spec that is not recommended? Anyway, it's all very well to say "It will be OK when warmed up" but you don't say the length of the typical journey. If it's a local commute, most of the running won't be up to temperature. Feb 12 at 23:31
  • Because most things I've read say things about ultra cold temperatures, improved fuel efficiency, but not much about any damage that might occur if I leave it in there until the next change (6 months). Maybe Subaru picked 0w-20 for no other reason than because it gave them an extra MPG. Your comment isn't helpful at all.
    – Spivonious
    Feb 12 at 23:40
  • You should still state the amount of cold-engine driving that is done, but you write it off as irrelevant, when it isn't, as your post implies. Feb 12 at 23:41
  • Honestly, there's a fair amount. My wife generally makes trips <5 miles. But if the 0w/5w is measured at -40F, then they're probably extremely close at +40F through to operating temperature. What is the viscosity of 0w/5w at 60F? 90F?
    – Spivonious
    Feb 12 at 23:45
  • So, the engine rarely gets to warm up properly in a near-freezing climate, so you've negated your "it's ok at normal running temperature." Presumably you changed the oil yourself to have made the mistake, so it's going to be inexpensive to change it again. Feb 12 at 23:47

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There are two reasons why you should change it at your earliest convenience, neither having anything to do with weather.

First, the weight matters for the purposes of variable valve timing setups. The VVT system (I believe your Subaru has what is called the Active Valve Control System or AVCS) relies on a specific weight of oil to ensure it activates correctly. If the wrong weight of oil is introduced, the system may think something is wrong and put your vehicle into limp mode, which would truly be a PITB.

Second, bearing tolerances are set by the manufacturer to a certain amount based on the type of oil which is going to go into it. Having the wrong weight of oil in the engine can cause damage to the engine over time. As oils become thinner, clearances become tighter (or maybe it's the other way around? ... which came first: the chicken or the egg?)

Are either of these issues going to cause major damage if you don't change it out in the next 500 miles? Probably not. However, it's a good idea to change out the oil to the proper weight as soon as possible to avoid any problems which might occur.

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  • Thanks Paulster. I could see using 10w or 15w instead of 0w, but 5w is enough difference in viscosity to confuse the computer?
    – Spivonious
    Feb 12 at 22:19
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    @Spivonious - I don't know "for sure", but would say it is possible. Most VVT systems measure the oil pressure at places in the VVT to ensure it is doing what it is supposed to be doing. If something is off, it could throw a code and make life miserable. Feb 12 at 22:23
  • I've found some charts on various sites comparing pumpability and viscosity between different weights. The difference between 0w and 5w is very small until you get below -5C, at which point it gets pretty significant. Since warmer weather is on its way, I'm going to leave the oil in until the next change. But this has all been very educational for me.
    – Spivonious
    Feb 15 at 18:00
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The oil would be thicker than the manufacturer recommended during startup, especially on the cold day; effects may show up during engine cold start when the oil would be thicker than the engine was designed to run with, thus making the oil harder to circulate throughout the engine. It's always recommended to use oil spec that's recommended by your car manufacturer for the best convenience.

Watch this video for better understanding about oil viscosity and it's impact on the engine: https://youtu.be/IKdhgKUZhPA?si=t1FyxwyP1vnudkRX

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  • Maybe, but the question is should the oil be changed.
    – Chenmunka
    Feb 14 at 10:35
  • Thumbs up for that video. So far, we've noticed no difference in perceived effort of the starter or engine on cold starts.
    – Spivonious
    Feb 15 at 17:58
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TL:DR - No, you don't need to change the oil.

The reason some manufacturers like Honda and Subaru have moved to 0w oils is for fuel efficiency, because 0w oils flow better when cold than 5w oils the resistance in a cold engine is lower, meaning a small gain in efficiency and a bit less wear at low temperatures.

The actual difference is marginal in your case, the operating temperature of 0w oils is down to -40°C (-40°F, not a typo, this is where the scales meet), whereas 5w oils it's -35°C (-31°F). As you are getting nowhere close to those temperatures there will be little difference in the viscosity of your oil when cold, and it could cost far more for you to replace the oil than you'd ever save in mileage.

If you lived in a much colder climate it would be more of a consideration, but I still wouldn't replace the oil just for that reason, it would have to be too close to the lower operating temperature of the oil.

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    This is the wrong answer. Why should they change it? Because with newer engines require certain viscosities in order for their variable valve timing correctly (ie: VTEC in Hondas). If the wrong viscosity is used, the computer can view this as an issue and pop a code which appears on the dash. This can also put the vehicle in "limp home mode" which can leave the driver in distress while trying to get their vehicle home at 20 MPH. Feb 12 at 19:16
  • I disagree @Pᴀᴜʟsᴛᴇʀ2, yes it would be better to have 0w in it as the manufacturers, however given the relative warmth of the climate where the poster is it's unlikely the difference between 5w and 0w is going to make a difference or cause damage to anything.
    – GdD
    Feb 13 at 8:59
  • You should really read my answer. My saying your answer is wrong has nothing to do with the weather, climate, or relative warmth. You state the OP shouldn't worry about changing the oil, but that is flatly wrong for the reasons I've given. Feb 13 at 11:04

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