How do I clean this goop?

My NOS Longines Seastar 7 has been sitting for about 30 years.

Over that time the caseback O-ring has transmogrified into a ring of thick black sticky goo.

I'll have to strip the watch to get rid of it as doing it in-situ would be suicide, this stuff stcks lick sh!t to a blanket.

I've taken small samples and tried to find out which solvent will break it down but no success, any tips are welcomed.

Have tried:
Methalated Spirits
Shellite
Kerosene
Mineral Turpentine

I've had good luck with isopropyl alcohol or rubbing alcohol.

Thanks 👍

I'll give it a try tonight, I didn't think to look in the bathroom cabinet 😀

I use my ultrasonic tank (for bracelets and cases) with a diluted solution of Mr. Clean. It will dissolve this black goo entirely and does not affect the case or crystal at all. Alcohol will also work, and I use the 99% pure variety for many jobs in the shop. It will take more effort than using the ultrasonic tank though.

Cheers, Al