Mixing synthetic and mineral Engine oil

At work, we have often had this debate, we get surpiled 15w40 mineral oil by a rental company for servicing their vehicles, but the customer who uses them give us 10w40 fully synthetic to top them up with. we have mentioned that it doesn't sound like a good idea, but as usealy they don't listen
the vehicles do a lot of miles, as you would think any logistic vehicle to. and we haven't had any mayor problems that link directly into it, so it isn't going to rapidly destroy any parts. it mite be different in a smaller engines tho. as this is in a 7.8TD,

I wouldn't recommend it tho, i would stick to one oil. it isn't going to do it any good.
 
stick with 1 brand, 15w40 is usally for older cars with high mileage, so its a bit thicker
syntetic is lighter, mixing the 2 makes no sense
 
It's nothing to do with the viscosity at all. It's to do with the additive pack used in the manufacture. I wouldn't mix two different synthetics under any circumstances.

Polyalphaolfin based lubricants don't work weill with polyolesters.
 
It's nothing to do with the viscosity at all. It's to do with the additive pack used in the manufacture. I wouldn't mix two different synthetics under any circumstances.

Polyalphaolfin based lubricants don't work weill with polyolesters.
your just saying it in a technacal way hdi
15w40 and 10w40 are different oils poly this and poly that

were both agreed its not a good idea to mix;)
 
Yes, but if the base stocks are radically different then you will have problems. Less so than just by mixing different grades.

Best avoided though unless you fancy ploughing through the technical datasheets for every oil you plan to introduce to another.
 
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