The International Cessna 170 Association

Says it's mandatory.
https://support.cessna.com/custsupt/con ... s_id=53745

SBs are not mandatory (in the FAA’s eyes) for part 91 operations. If the problem persists, it could become an airworthiness directive, in which case it would become mandatory. That said, I don’t think it would be a big deal to strain drain oil through window screen material and see what you can find. Also, the chunk of the size shown in the photo should be caught in the oil pickup screen.