I understand your concern about Naphtha, but my real life data doesn't follow that at all. I use Naphtha EVERY DAY to clean rubber. I fix over 500 games a year, and on every one, I use Naphtha to clean the rubber. That's a lot of rubber cleaning, and on a big assortment of different brands of rubber. I also "collect" rubber (i have a large assortment of different NOS rubber brands and types, collected over the last 15 years), and find different brands work better/worse in certain games. I have about 10,000 rubbers rings in stock at any time. So i'm pretty versed in rubber types, and I've found that naptha works great to clean all of them. Also rubber pinball rings are not natural rubber. In fact I don't believe they contain any (or very little) natural rubber.
The bottom line is this... keeping rubber dirty sets up a chemical reaction that degrades the rubber. Using naphtha (or pretty much any other decent cleaning product mentioned in this thread, except wd40) to clean the rubber is going to extend its life. Again i've used just about all the rubber cleaning products over the years, and lighter fluid (naphtha) is by far the best, most readily available, easiest to use, and cheapest. It leaves the rubber clean and bouncy (assuming the rubber hasn't degraded too much due to dirt and UV light.)
The application may also be a factor. I squirt a little on the edge of a folded paper towel, and wipe the dirt off the rubber. That's it, quick and easy.
On wd40, i would also avoid this on rubber. after the solvent in wd40 evaporates, it leaves a gummy reside behind. I think it's the solvents in the wd40 that's cleaning the rubber, but the other junk in wd40 that gets left behind is what would concern me. So in my eye, avoid the leftovers, and just use the solvents to clean.... Naphtha!