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.