I'm sorry guys, i'm with Andy on this one, why go into all the bother of making one side shiny and one side matt if it wasn't for a reason
Not changed the oil myself yet (but will be soon so need to know)
Andy this is how my mind would approach this issue ......
I would say if there is a shiny side and a matt side there has to be a reason and it ain't to look good.
Presumably the sump drain hole is machine finished, which would give the better seal the smooth flat side or the rough matt side ?
Personally on this I would go with the Shiny to the sump.
But.....
Is the shiny side just there to tell you the washer has been used before and you need to use a new one?
Maybe the old washer would give an indication of which way it was fitted.
But .....
What happens if the old washer was put on wrong?
Has anyone checked the handbook BTW