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Dear Car Talk: My Audi requires 5W-40 oil. I use Mobil 1, full synthetic.
I have some 0W-40 left over from a previous vehicle. If I mix equal parts of 0W-40 and 10W-40, will that make 5W-40? — Ralph
Probably. I don’t have a certified viscosity lab in the garage, but I would imagine that you’d get something in between.
In any case, it’s very unlikely that any engine damage would occur, even if you mixed in a little bit of a lower viscosity oil on its own.
The label 5W-40 means that the oil acts like a 5-weight (thin) oil in cold temperatures. That makes it easier to start when the engine and/or temperature are cold. It also improves your mileage since the oil induces less friction. The 5W-40 acts like a 40-weight (thicker) oil when it’s hot. So, it affords better protection in hot weather and when the engine is running at operating temperature.
I’m guessing that Audi determined that they didn’t want a super-thin oil, like a 0W-40, in colder conditions. Maybe they were concerned it wouldn’t provide enough protection to crucial engine parts. After what was presumably a lot of testing by concerned German engineers in white lab coats, they specified 5W-40 for your car.
So, before you try to save 40 bucks worth of leftover oil you have on the shelf in your garage, you might want to think about how much that Audi cost you. And how much an engine repair costs at the Audi dealer.
Is it worth experimenting on?
I mean, you could just use the 0W-40 in your lawnmower. Or put your three leftover quarts on Craigslist for half price, and someone whose car uses 0W-40 will snap it up in an hour.
So, it’s up to you, Ralph. I think the chances of doing any damage, if you experiment, are extremely low. But it’s not my Audi.
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