...grease build-up in the plumbing
Actually, through my readings about The Home Front, during WWII, the grease was collected for something. Too much Scotch has temporarily clouded my memory, but I think it had something to do with munitions manufacture. The grease was collected by the same guys who took away your legacy heirloom bicycles, pots & pans, oil lamps and Victola phonographs for the war effort.
Other FReepers can fill in the details...
There probably was a collection for it, I’d guess my parents were too young to know.
I actually never questioned it until one of my housemates asked why I did it. The grease-build up came to mind (it might not even be an issue), so I went with that.