A quick google search tells me that ‘normal engine oil’ is non conductive. So I would guess that you can trust the manufacture with his advice.
(A lot of text but this is one of the search results: Is Engine Oil Conductive? A Comprehensive Guide – Oil Genesis)
Another search got me to this forum where they are talking about a different version of a Hanning pressure washer. However, as far as I understand it, the selling point of Henning is that they use a submerged motor en pump that is the same to many of there models.
In this post the person says it uses just “Einfaches Motoröl” which translates to “Simple motor oil”. (but you’re probably German so you can read and understand that topic better than me)