Nowadays mixing oils equals mixing a lot of aditives.
This may mean LOTS of trouble.
So, there is no simple answer.
I'd say: Use either the manuf. spec or better.
Probably the best compromising is a semi-synth - a kind of technically made mix. (btw semi-synths may appear in the manuf. lists)
Jorge

PS: When I mean better I mean: better but included in the manufacturer list. Because of sludge many manufacturers are moving toward synthetics. I am moving from semi to full - concluded that the price ticket is not that big and it is always a cheap insurance policy vis a vis servicing a MB.