I've had my H6 from 2005 and in the early days there were a few that had engine failures. When the engines were new everyone said to use mineral oils to help the bedding in process. After around 20-30,000 Klm's you could switch to part/fully synthetic oils as the engine would have been bedding in by then.
As most H6's on this forum are not new and possibly second/third owners fully synethetic may be the way to go. Which ever route you take, mineral or synthetic there is one thing that needs to be done regularly, CHECK YOUR OIL LEVELS.
By the way up to 30.000klms I used mineral oil, then switched to part synthetic and now I use fully synthetic and I have no excessive oil consumption. But I have had this car from new.
Rob