I have used both copper and Dowty washers in many situations in the past. Of the two I prefer annealed copper because it does not decay like the neoprene O'Ring in a Dowty washer. Many years ago I had to fight a fire in a very large air compressor, a dowty washer had been weeping cooling oil onto the internal electric drive motor. The vertical chimney and cooling matrix was also full of cooling oil. When it caught fire it went up like an atomic bomb. Four engineers managed to control the blaze until the fire fighters arrived. Hundreds of thousands of dollars of damage!
Bob