I strongly suspect you'll need an 8mm square adapter.The best sort to get are either the type that are a socket or ones that you can use with a spanner.The universal 8 in 1 adapters are,in general,not up to much.
When I changed the oil and filter on my lag recently the sump washer was a steel one and not copper, the local Renault dealer was feeling generous when I bought the oil filter and gave me a steel and copper washer free of charge is there any difference between the two washers apart from the obvious answer
What you are referring to as a steel washer is in fact called a Dowty seal. These are used in the main for industrial hydraulic systems. As has been said it has a rubber insert which was not designed for the temperatures involved with the car engine sump. In short you should only use the copper washer which will not be affected by the heat.