My less than 15,000 mile when I bought it K100/2 turns out to have hidden damage.  After a couple of thousand miles I decided to do another oil change.  After all, it had stood for over 20 years all together.  This time, I just happened to have a good look up inside the sump.

Hmmmm, the sump wall looks a bit damaged, someone must have had a job getting the oil filter off.  Looking closer, I could further see that there were two pieces broken off the edge of the oil filter seat and part of the seat is actually indented, again mostly towards the outside.  There are also various chips and knocks on the seat's surface.  The bike's been running fine, oil light stays off, so sealing must have been taking place.

Off with the sump plate to clean things up a bit, in case there were bits of alloy floating around.  Oh look, someone has levered this with a screwdriver (yet it wasn't leaking oil).  Where the outside of the filter seat is chipped, the little outside lip was also bent inwards, so I've cleaned this up a bit.

I may not be the world's best mechanic, but whoever has been at this should never touch a spanner (or a hammer and chisel) again.  Let's hope it continues to seal.  I don't have the space, or the inclination, to undertake the work involved in fitting a replacement lower sump.

Oh, if I were to sell the bike in the future, you should all forget this post    (Honest guv, it were fine when I sold it!)

Sounds like someone tried to hammer the oil filter out using a screwdriver instead of buying the removal tool.

I've taken note just in case you do put the bike up for sale.

It looks like you and your bike have had a lucky escape-enjoy the bike anyway.

Noted!!!