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I have an old gasoline car which require SAE 15w-40 engine oil, but such an oil for gasoline engine is not available in my country.

I recently found some diesel oil which has API CJ-4/SN or other categories with sl/sm at the end and has SAE 15w-40 grade. does this mean i could use them in gasoline engine? if not, then what does it mean?

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  • What is the minimum Celsius temperature you expect in the coldest month in your region? And the hottest temperature in the warmest month?
    – MTA
    Dec 31, 2022 at 22:14
  • @MTA about -10 to 35 degrees.
    – Mohammad M
    Dec 31, 2022 at 22:38

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Your annual temperature range is -10C to 35C.

The oil viscosity of 15W-40 means that the oil is no thicker than straight SAE 15 oil at winter temperatures and no thinner than SAE 40 weight oil at 100C. Winter temperatures are defined as follows:

0W is -30C; 5W is -25C; 10W is -20C; 15W is -15C; 20W is -10C; 25W is -5C

Summer temperature is always defined at 100C for all viscosities.

This means that if 15W-40 for gasoline engines is not available, you can use 0W-40, 5W-40 or 10W-40. You will probably find that 10W-40 for gasoline engines is widely available and lower in price than the other grades and lower priced than diesel oil too.

If you check the manual for your car, it will most likely say that 15W-40 oil is recommended for all conditions and it will give a graphic chart that shows other grades of oil that may be used at specific temperature ranges. I think you will see that 10W-40 is approved for use in your temperature range of -10C - 35C.

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  • Thank you, Considering the last owner used 20w-50 and also wear and tear on engine and recommendation of some mechanics, currently I use 20w-50! , rather than 10w-40!. but there are some knock issues which didn't solve by increasing fuel injection or retarding the spark to its limit (it has EST(electronic spark timing) where coil sparking controlled by ECM and the distributor has a constant advance which could be adjusted manually), so i was thinking by using a thinner engine oil maybe i could reduce the compression and sooth formation to alleviate the knock issue.
    – Mohammad M
    Jan 4 at 8:40
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API SN is the category for gasoline (petrol) engines. According to API website

SN is as follows:

For 2020 and older automotive engines

SM is as follows:

For 2010 and older automotive engines

SL is as follows:

For 2004 and older automotive engines

All three are current. While the 15W-40 is primarily meant as a diesel oil, if it has the API SL/SM/SN categories listed as well, you can use it as such without issue depending if your vehicle falls within those particular years.

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