A lot happens in nearly 30-years! Manufacturers are only 'required' to keep parts around for 10-years on certain things, although the design may be common, and they may keep things. Any rubber parts could be expected to be toast in that timeframe. Are you talking about the seal between the bottom of the tank and the bowl? On many toilets, at first, it seems like they are too thick, and you could never get the tank tightened down enough to make it come in contact with the bowl. Usually, though, you can. First, it often works best if you use four nuts on the tank/bowl anchors. Under the head of the bolt, use one rubber washer in the tank, then on the bottom of the tank, use one metal washer and a nut. Tighten that up firmly so the that it creates a good seal in the tank. Then, use a second nut and washer to anchor it to the bowl. WHen tightening, keep the bowl square and plumb while tightening the two nuts evenly, a little bit on each side while alternating from side-to-side. If the proper gasket is used, normally, the tank will be able to come into contact with the bowl and be stable. If you don't feel comfortable doing that (or the gasket really is incorrect) and it doesn't leak, you may need something there to keep it from wobbling. Many of the newer toilets have little spacers or legs on the tank that are supposed to make contact with the bowl to keep thing stable. If the use of a second nut and washer on the tank would raise the thing too high so those (if they exist) can't make contact, then you need something else in there to take their place. Be creative...it doesn't really matter.