Any issues with mixing engine oil?

ultim

ultim

ultim

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Hi,

My car is low in oil. I have 1/2 litre of 5w40 and 15w40 laying around.

Is it wise to mix the oil?
 
No harm at all mixing engine oil grades for a temporary top up, especially if it's only +1 or -1 grade away from what the engine is supposed to use.

5/40 and 15/40 both have the same viscosity when hot.

From Shell.ca:
"If you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product should work as expected. But there's absolutely no danger about incompatibility resulting from mixing engine oils,"
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As said above, if you are only topping up then its OK to use a similar oil.

It actually states this in the Haynes manual for a Ford Fiesta, where the oil recommended is 5w30 and it says that 10W40 is OK for top ups.
 
( |-| |2 ][ $;14340193 said:
Really you should be using the grade the manufacturer states you should use.
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Unless you own an RX8 in which case, you will spend the next six months trying to get a consensus on which oil type Mazda really had in mind for it :D
Seems to be 15w30 fully synthetic at the moment BTW

:D
 
Topper said:
Unless you own an RX8 in which case, you will spend the next six months trying to get a consensus on which oil type Mazda really had in mind for it :D
Seems to be 15w30 fully synthetic at the moment BTW

:D
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Good old RX8s. All they need is oil low in viscosity modifier and no PAO as these oils burn cleanly.

Its the deposits that cause problems - deposits from burnt rubber and highly thermally stable base oils.
 
many oils can be mixed together but not all. if they are made by the same manufacturer then contact them. i have done this a few times and been Very surprised at how helpful they have been.
 
ressurecting this a bit but why is it ok to mix oils just for topping up but not for changing oil?, I have 2 litres of 10w40 and half a litre of 5w 30 both same manufacturers why cant I use it for my bike? I wanted something a little bit thinner than 10w40 but didnt want to use just 5w 30? why cant I just use the two?
 
Rich1988 said:
ressurecting this a bit but why is it ok to mix oils just for topping up but not for changing oil?, I have 2 litres of 10w40 and half a litre of 5w 30 both same manufacturers why cant I use it for my bike? I wanted something a little bit thinner than 10w40 but didnt want to use just 5w 30? why cant I just use the two?
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Would you mix blood into it?

The viscosity is controlled by the additive packages of the oils. Mixing them will give you unpredictable results. In maths 10 minus 5 is 5 but mixing chemicals isn't necessarily based on simple maths. As I said, you will get unpredictable results.
 
mejinks said:
Would you mix blood into it?

The viscosity is controlled by the additive packages of the oils. Mixing them will give you unpredictable results. In maths 10 minus 5 is 5 but mixing chemicals isn't necessarily based on simple maths. As I said, you will get unpredictable results.
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Where did mixing blood into it come from? Totally flawed analogy, engine oil is intended for use in engines.

The results won't be that unpredictable. As posted above, mixing 5w30 with 10w40 will give you an oil that performs somewhat better in the cold than 10w40, but less good cold than 5w30, and that has a viscosity a bit higher than 5w30 but a bit lower than 10w40.
It's harder to predict where exactly the cold performance and viscosity will be, but it will be something between the two oils.
 
Rich1988 said:
thank you! i trust you more because you said what i wanted to hear! :D
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lol :cool:

Actually I mixed engine oils recently anyway, so I'm biased. I was working on the principle that there was at least 2 litres oil in there to start with (probably nearer 3), I had a litre of spare oil at 5w30, and according to Haynes the car takes 10w40-10w50, or something like that. Didn't have a clue what was in there already as I hadn't topped up/changed oil before, but I figured the 5w didn't matter, and the decreased viscosity of the new oil would be totally offset into something peugeot-palatable by whatever was in there before. Very bad of me of course, but she's running better than she was when approaching the minimum mark..

Anyway by my reckoning, decreased viscosity would mean more likely to burn/lose oil? Whereas higher viscosity could mean more chance of the engine seizing?
 
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