[katy]

One has to be careful using plastic for drain plug gaskets. One time I had the modern car serviced at a "quick lube" place, a few days later I noticed a small oil leak at the plug. On closer inspection I saw that the gasket looked odd. I took the car back to the "quick lube" place and watched while the attendant removed the plug and the oil. The gasket had partially melted and deformed enough to leak. The gasket was replaced w/a copper one, fresh oil added and no more leak.
So, if using plastic for making oil drain plug gaskets, make sure it can "take the heat". Don't forget that the oil in an engine runs 20°-40° hotter than the coolant.