I agree that the oil cap seal is a cheap part and worth just replacing. If you don't have time to order one and just want to move forward, you can also just cut a gasket the same size and add it behind the original so it's tight again to stop the leak. You can always wait and order that after when it's convenient for you.

Most O rings are Neoprene, I think its more resistant to oil than natural rubber like from rubber trees. Natural rubber has some certain properties, for example it is used on pool tables where the bounce is important and it has a more grippy texture so that can help if it's a roller used to drive things like paper.

There are a bunch of different synthetic compounds. Butyl rubber is one. Natural rubber has a shelf life , eventually it goes hard and breaks down. Its probably less oil resistant than most of the synthetic rubber products of today.

the natural rubber needs a different process, some of the synthetic rubbers are easily molded and cured hard with heat. I worked at a place that made printing press rollers, it came in drums and we put it in molds , cured it in an oven than ground it to final dimension.

here's a pretty informative short video that shows natural rubber being made and extruded for pool table cushions at the Brunswick factory.

https://www.youtube.com/watch?v=qyiDDhNaczw&ab_channel=kdmunroe