As I go through an engine rebuild during a restoration I'm adding o-rings everywhere I can. So many of these classic engines and gearboxes had no seals and the resulting oil leaks, which were allowable before the advent of oil-tight Japanese motorcycles, are now verboten. So places like rocker inspection caps, kick start shafts, 1" primary/gearbox inspection caps, all get o-rings. If there's no gap where an o-ring can be fitted, then I'm forced to modify the shaft or cover to accept one.
Just one of the places today's engine builder has to update the old parts to meet a specification that has never been spelled out, but is always expected.