I live in Chicago where the night-temperature averages in low 20-degrees-fahrenheit and last year’s (2019) record low went to 23-degrees below zero. To prepare for another (possible) year of low temperature, I replaced the engine oil in my generator and snow-blower with synthetic 5w-30 (API SN).


  1. Is synthetic 5w-30 good enough or should I use synthetic 0w-30 instead?
  2. What is the lowest (reasonable low) temperature that synthetic 5w-30 can handle?
  3. Can I leave synthetic 5w-30 over summer since (theoretical) specifications say that it can be used for temperatures as high as 90-degrees-Fahrenheit
  4. What is the downside of synthetic 5w-30 compared to synthetic 10w-30

(BTW, my carburetor is perfectly clean and I have no problem starting at around 30-degrees-Fahrenheit)



3 Answers 3


Ideally the manufacturer has engineers that put a lot of thought into this, and the manual spells out all the recommendations for even the most extreme operating conditions. But, lots of times you don't get that, it just says "use 10W40" and that's it.

If I think my operating conditions are likely to be way out of what the manufacturer's recommendations account for, I might deviate from those recommendations. I might use a more viscous oil to deal with the heat, or less viscous oil to deal with the cold. Your conditions in Chicago are typical for huge parts of the country, I bet the manufacturer's recommendations are fine for Chicago. I'd probably leave well enough alone and stick to the recommendation.

If the engine has a lot of miles / hours, and seems to have significant wear, I do switch to something a little more viscous. If a car is burning more than say a quart between oil changes I will suspect it's worn. I figure an engine with some wear is a little looser and will blow by a little more oil (past the pistons into the combustion chamber) so a more viscous oil might make sense, like switching from 10w30 to 20W50, especially in the summer months. I have seen this make a measureable difference many times with high mileage cars.

I haven't really seen the extra viscosity make much of a difference in starting in cold weather. I think the drag you get from viscous oil is pretty minor, better to focus on a good quality battery with adequate cold cranking amps and good cold weather performance if you're concerned about hard starts in cold weather.

  • 1
    Thanks. I checked the manual and it said 5w-30 is good. However, in another section it showed 0w-30 for temperatures below 20F (and 23F seemed to be on the edge). That's what got me thinking about 0w-30. My snow-blower and generator are not used much so they have little wear. Guess I can leave 5w-30 in them through summer too. Thanks for your detailed and highly informative response. Appreciated it very much!!!
    – user97485
    Commented Dec 21, 2019 at 17:20
  • The winter weight (0W, 5W) is the viscosity of the oil when cold, both recommended multi-weight oils contain polymers that will increase the viscosity to the same level when heated. The reason the manual specifies a lower winter weight for colder temperatures is to allow the oil to flow while the engine warms up. It's more of an issue in large engines, but Alaskans and Canadians may need to use a snow blower when it's substantially below 20F. Commented Dec 21, 2019 at 20:58
  • 2
    Matt: Thanks for elaborating on the reasoning behind the fundamental difference between 0w & 5w. The point they made in the manual wrt 0w now makes sense … since they were perhaps addressing Alaska/Canada use-cases. 5w seems good in Chicago. Update: I ran my engine for 15 minutes and then drained the oil completely (tilted it too!!). After adding 5w-30 (and with a clean carburetor), the snow-blower and generator both start very easily with 1-2 pulls at about 30F
    – user97485
    Commented Dec 22, 2019 at 16:34

Any oil with a 5W rating is good down to -35 deg C, which is -31 deg F. Therefore based a one-time low temperature -23 deg F, you should be fine with 5W-xxx.

  • Steve, thanks!! That's what I thought based on internet research; however, I wanted to confirm it with folks more knowledgeable and hands-on than myself.
    – user97485
    Commented Dec 21, 2019 at 17:14

The W rating in oil viscosity is tested at 0° F (-17.8° C). The following number (after the W and the "-") is tested at 212° F (100° C). So basically, a 0W-30, a 5W-30, and a 10W-30 all are in the same viscosity class at 100° C and will operate similarly. Most engines are designed to operate at a relatively narrow temperature range after warming up. Air-cooled engines are not as narrow in this regard as water-cooled engines but even at very low temperatures, your oil will not stay cold during operation, so the 2nd number is important.

The most important thing about the W or 1st number in the rating is the ability for the oil to be pumped and lubricate the engine parts quickly after a cold startup. In addition, heavier oil will put more drag on the starter and make starting more difficult (especially important if the starter is YOU).

There is very little negative for the 0W in a 0W-30 vs. 5W-30 comparison. Typically the 0W may be a little more expensive. When you get into the details, the 0W may have a higher content of "viscosity improvers" in order to get the bigger range between the W and non-W viscosities, and this higher content of viscosity improvers can make the oil slightly less effective in basic lubrication, but that is normally an effect so small as to not be considered.

To make sure I hit your list:

  1. I'd personally be comfortable with 5W down to 10 F or so but for the extreme stuff, I'd opt for the 0W-30
  2. There is not a hard and fast rule, but again, I'd draw the line somewhere in the 0-10°F area as where I'd really rather be using 0W.
  3. Yes, you can leave 5W-30 (or even 0W-30) in the engine in the summer as the heated up viscosity (the 2nd number) is basically the same.
  4. Noted in text re: price and higher viscosity improver content.

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