Dowty washers as suggested by "the hobbler" are great
, however I'd suggest checking a couple of other things.
1. Try screwing the drain plug in by hand without any washer fitted and check that the plug seating surface can make contact with the sump without running out of threads - (the thread on some drain plugs isn't cut all the way up to the underside of the plug head) - a washer of suitable thickness is needed to avoid this happening.
2) Check that the hole tapped in the alloy sump is at 90* to the sump sealing surface - if not, the plug can be tight but the washer is not being clamped all the way around evenly and may leak. A rough check on perpendicularity (sic) can be done by screwing in the plug most of the way and then measuring the gap where the washer would fit at intervals using feeler gauges. If it's a 'little off', you might get a good seal by using a more compliant type of washer material e.g. fibre or aluminium covered with fibre (as used on some Toyota's for example).
3) Copper washers as supplied are often 'too hard' to conform to the surfaces being sealed by normal moderate tightening - I've experienced this with new copper washers used on both sump plugs and brake hose unions. Copper both 'age hardens' and 'work hardens' but thankfully can be easily annealed (softened).
To soften a copper washer, heat the washer to a dull red heat then quench in water. This heating can be done using a gas blowtorch, camping stove or a gas ring or similar. Normally, to anneal a metal, you'd heat to a red heat and then allow to cool slowly in air, this will work on copper also, but copper is different to other metals in that it can alternatively be quenched in water - the reason I'm suggesting quenching in water is that this will remove any scale that results from heating it to red heat. You might also notice that the annealed copper washer when quenched in water has a slightly pink hue to it and can be easily bent using finger pressure, unlike an un-annealed washer as bought. I routinely anneal most copper washers before installation.
If Dowty washers are not available to hand, you could try using a copper washer with a larger size centre hole and insert a rubber O ring to take up the clearance on the drain plug, if the rubber is slightly thicker than the washer, it should seal well with normal moderate tightening - this might also work to seal a drain plug where the tapped thread is not exactly perpendicular to the sealing surface. If using this dodge, best to keep a check at intervals that the drain plug hasn't loosened in use. Please don't be tempted to use this dodge (bodge?) anywhere on brake system connections/unions or any safety critical applications.
