On the top of the upper gear case there is a oil dipstick. There is a seal there below dipstick screw and the one on my boat is molded to fit in the area below the screw (dipstick) Now I went to the shop and they gave me a flat rubber washer and I told them "that was not what was on there now" and they said that "the flat ones are all they have seen." Is a flat rubber seal a suitable replacement and what do you use? Thanks