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Some folk will say that you shouldn't mix different oils, and I can see where they're coming from, but I personally don't see any particular harm in doing so if your car is fairly standard engine wise. I take it you were thinking of a semi-synthetic oil? I guess thats what most people would use in their 5 unless they have a turbo or supercharger fitted.
Having said that, it won't cost much more to drop the oil & fit a new filter whilst you're there, then you won't have the problem of not knowing what oil is in there.
 

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I would change the oil personally, but i can't see why topping up with 10W40 semi could hurt.
 

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mixing oil is fine. I would change the lot though unless you know for an absolute fact that its new. Which at 3000 miles its not really.
Its only a bolt. The filter is more involved, but the oil is quite easy.

chris, you would normally run plugs a grade cooler. I think this means they stay cooler by passing heat from themselfs to the head matterial quicker than a hotter plug. Thus one is hotter than the other. Oil wise its not far from personal choice. Generally fully synthetic, but i 'imagine' the bbr ran on a 10w-40 semi.
 

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when was it last serviced? if you don't know, it's probably worth doing an oil and filter change.

otherwise, just put some oil in it. was the standard oil spec even semi synth? semi synth will be fine either way
tom
 

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You might as well change it just because of the mileage ad 'unknown quantity' factor , but I wouldn;'t worry about mixing types too much. Sounds like you'd be topping up about 0.5 litre, which is probably not far short of what hangs up in the engine when doing a change anyway, and I suspect most people don't take off their sump and wipe it out and dry out all their oil system when they drain and refill with a different type.
 

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Got a similar question to the above.

Don't know what is currently in there and have two weeks until car is in for a service. Would some 15/50 f/s mobil 1 be ok or should I pick up some 10/40 s/s to carry me over?
 

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A 0W40 will still be much thicker when cold than a 5W40 oil is when warm, so if you were going to see any problems with too thin oil they would occur when the engine is warm, when it be the same viscosity as a 5W40 anyway.
 

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yup, oil loss is closly related to the viscosity at high temps - and in that regard a 10W-40 is the same as a 0W-40.

as andyk says, all oils are much thicker whn cold than when hot so all of this 0W-X being too thin needs to be taken with a big pinch of salt, it's still many many times thicker when cold than a nice thick 10W-60 is after 10 minutes of driving.
 
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