Oil weight question
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This is a discussion on Oil weight question within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; Ok, so I've read the FAQ's about oil and all but there is one question I still have. Right now ...

  1. #1
    Registered User Mark_02WRX's Avatar
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    Oil weight question

    Ok, so I've read the FAQ's about oil and all but there is one question I still have. Right now I have 5w30 Valvoline oil in my car, I was told to run 15w40 due to the high temp summers here in AZ which can get up to 110-115 degrees. I know there is usually a drop in MPG with going with heavier weight oil but the question I have is ROUGHLY (Because I know there's no definite answer) how much of a drop do you usually see when you factor the weight of oil and outside temp when going with heavier oil?? I would imagine it's not that big?? Maybe 3-4 mpg loss??

    TIA,
    Mark

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    Moderator T0rque's Avatar
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    Same summer temps here, I was running mobil 1 5w30 for a long time and my mobil oil rep told me to run the mobil 1 racing 4t 10w40 motorcycle oil. So I run that now.. Have for awhile but not because it's hot


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    Registered User Mark_02WRX's Avatar
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    Yea the oil I was told to use was 15w40 for diesel motors. I just want to know if I'm going to see a BIG drop in MPG with switching to 15w40 from 5w30.

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    Moderator T0rque's Avatar
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    I wouldn't use 15/40.


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    Registered User Scuba2708's Avatar
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    i personally use 5w40 rotella t6...its full synthetic and it has the 40 weight part to keep its thickness when its hot but it will still flow quickly through your motor on startup which is a very good thing.

    according to my AP i get an average of about 29 mpg, this is at stg 2 driving through back roads all the time...im too lazy to actually do math and calculate my mileage tho lol
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    Registered User wrx wagone's Avatar
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    I didn't notice any loss when I switched to 5w-40 Rotella.
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    Registered User economatic's Avatar
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    Just regurgitating what the owner of a reputable Subaru shop in the area said...

    For our climate in the DC area run in a flat 4 at least 5w40 but 10w40 is better. He said he has no problem with 15w40 although it will start hard in the winter. As for the reasons for 15w40...

    FYI, I have no expertise in engine oil but from my experience my '09 would drink 5w30 when driven hard. I think it was Valvoline, whatever the dealership used. I switched to Motul 5w40 and the oil on the dipstick hasn't moved in 2,500 miles. However, you can take this with grain of salt as my car has low mileage.

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    Registered User Mark_02WRX's Avatar
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    Sounds like some good advice from all you guys! I think the next time I do an oil change I'm going to use the 15w40 due to the higher temps here in AZ.

    I figure that if you can use 10w40 in DC with no problem and it gets to about 90-95 during the summer (I'm guessing) then I should probably step it up with the 114-115 degree summers here lol.

    In the winters I'll just switch the oil weight back to 5w30 since it gets pretty chilly here too lol.

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    He simply abides. SD_GR's Avatar
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    I would not use 10W-40 in DC. The last time I was in DC it was below -10C.

    I would also not use 15W-40 in AZ, though I probably could. If you want to stick to Shell try the T6 in 5W-40 IMO. In fact, I use a 5W-40 or a 0W-40 in SoCal, and the ambient temps in my area are from 5C (winter before dawn) to 45C (summer mid day).
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    Registered User Scuba2708's Avatar
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    yea the 15 winter weight is too thick on startup for our cars unless its been rebuilt with looser tolerances. the 5 winter weight in 5w40 will still allow flow on startup but the 40 weight will still be thick enough for the arizona heat. only difference between 5w40 and 15w40 is that its the 15w40 wont flow well on startup
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    Registered User Mark_02WRX's Avatar
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    Quote Originally Posted by Scuba2708 View Post
    yea the 15 winter weight is too thick on startup for our cars unless its been rebuilt with looser tolerances. the 5 winter weight in 5w40 will still allow flow on startup but the 40 weight will still be thick enough for the arizona heat. only difference between 5w40 and 15w40 is that its the 15w40 wont flow well on startup
    So if I put 5w40 in versus the 15w40 the ONLY difference will be how smooth it'll be on start up? The 5w40 will be just as thick as 15w40 when the car is at operating temp in the summers here??

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    Registered User Nigel79's Avatar
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    Geez. Just worked a long day and this thread is exhausting. Ill have to read it later 5-30-40-10-15- hot cold
    '09 Sti

  14. #13
    He simply abides. SD_GR's Avatar
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    Quote Originally Posted by Mark_02WRX View Post
    So if I put 5w40 in versus the 15w40 the ONLY difference will be how smooth it'll be on start up?
    The differences will be oil viscosity, and so oil flow and pressure at startup. This implies differences in wear rates for the motor; I have not seen data on how significant this will be etc.

    The 5w40 will be just as thick as 15w40 when the car is at operating temp in the summers here??
    The 5W-40 and the 15W-40 will fall within the same range of viscosities when they are both new and they are both in a motor that has reached operating temp. As each oil ages their characteristics may change and these changes may not be similar. However, for the sake of discussion they can be considered equivalent when at operating temperature.
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    Registered User Scuba2708's Avatar
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    Quote Originally Posted by SD_GR View Post
    The 5W-40 and the 15W-40 will fall within the same range of viscosities when they are both new and they are both in a motor that has reached operating temp. As each oil ages their characteristics may change and these changes may not be similar. However, for the sake of discussion they can be considered equivalent when at operating temperature.
    or in simple terms: yes. lol
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  16. #15
    Registered User Mark_02WRX's Avatar
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    Well I have a much better understanding of oil now loL!!

    I'm going to go with 5w40 the next time I do an oil change

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