You are absolutely right, there is no reason to put teflon tape or paste on a straight thread where an O-ring is used to seal it. For the people that do it, Think of it this way. Everytime you put teflon tape or paste on the drain plug and screw it in, you are pushing some that was used before up into your engine,/quote]
I've read somewhere that if you feel you must use teflon, use paste instead of tape. The reason is that the stringy pieces of teflon tape that results when you remove the plug can find its way into the oil system and plug the piston cooler orfices. I have no idea if that is true or not. I don't use teflon at all because the o ring is designed to do the sealing, so why seal the threads.