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I have a 1994 Buick Century with a 3.1L V6. The cap on the oil reservoir calls for 5W-30 oil.

I know that the engine is full of 10W-30 oil.

I know that the number ahead of the "W" is an indication of viscosity at very low temperatures. With that knowledge and given the fact that I live in Florida, where the absolute coldest temperature I've ever seen in my 34-year life is 18°F, I don't believe this is a problem but I don't know for sure.

I'm inclined to believe that the cap is stamped "5W-30" because Buick doesn't know if the person who buys this car will live in Maine or Florida and they want to perform some level of "CYA" as it were.

So what want to know is if this is something I should correct. Should I immediately change the oil; should I change it back to 5W-30 in time with my next regular oil change; or can I just continue to use 10W-30 oil?

If additional information can help with a verdict; the engine has relatively low mileage (~98k) but it has been neglected (it was recently run for several months to a year with a leaky intake manifold gasket where coolant was able to contaminate the oil).

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    5w has the same viscosity at 0C as 10w has at 5C. And the difference is minimizing with the rise of the temperature.
    – oryades
    Apr 13, 2017 at 14:03
  • Yes I know Toyota service manuals call for 5w30 but they use 10w30 in winter environments where the temperature doesn’t get lower than 5 Degrees Celsius or 41 Degrees Fahrenheit.
    – milflight
    Sep 15, 2019 at 7:04

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Especially given that you live in Florida, 10W-30 vs 5W-30 is not an issue at all. Sometimes thicker oil is even recommended in higher mileage engines, although I believe the jury is still out on it. If you lived in Siberia, I would be a bit more concerned about it.

However, the coolant in the oil definitly needs to be addressed. If it gums up your oil pump or an oil galley someplace, you could end up with an actual problem on your hands. Fix the intake gasket and change that oil before driving it around anymore.

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  • Thanks for the advice. I had a mechanic replace the gasket a few weeks ago because I felt the process was more involved than I was comfortable with, and had him replace the oil and coolant at the same time. He filled it with 10W-30, though (or at least that's he put on the invoice) and it's what led me to ask this question.
    – Unknown Zombie
    Apr 13, 2017 at 19:59
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With temperatures going down to 20F a 10W oil will not be an issue. Should you be faced with that kind of "extreme" cold you can just let your car warm up for 30 to 60 seconds before you start rolling.

There is a small benefit of using a 10W instead of a 5W oil in that the 10W will normally have less viscosity improvers in it. Viscosity improvers help the oil behave "thicker" at higher temperatures, but they do not actually provide lubrication. So for example, a straight 20W oil or 30W oil with no viscosity improvers in it would have more lubricating oil than a 5W-20 or 5W-30 oil.

So in short, given your climate, I see no harm, and perhaps a small benefit in doing what your mechanic did.

ps: If you take a winter vacation to Fargo, ND, I would suggest that you have your oil changed before the trip, and in that case I would specifically ask for 5W-30. However, even if you end up on Fargo unexpectedly, just give your engine a minute or two of idle time after start to let the oil limber up as it flows and warms up.

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