Hi Shaun, I've used both copper and fibre and on one occasion purchased some washers that were copper edged and fibre ,they worked well but only once . Don't be heavyhanded with the spanner or you can bow the banjo and the oil will pee everywhere .Oh and periodically check 'em they do loosen and has I found out yesterday the oil spreads over everything ( I'd forgot to check the buggers ) Best of luck ,BobH.