Penetrol is a mixture of solvents (naptha, stoddard, mineral spirits) some linseed oil and some soya alkyd resin. When your paint gets dull, it's because the paint's surface has oxidized. By rubbing it down with Penetrol, you are essentially oiling that thin film of oxidized paint. It has a similar effect to oiling a piece of wood - the color darkens a little, the grain becomes clearer and you add a bit of shine to it if it's smooth enough. Same deal here, except the re-oiling is helping return the oxidized paint surface to a look more like it had when it was new and not oxidized. I imagine that the Penetrol actually soaks into the paint a bit, feeding it fresh oil and making it a teatment that's more than just sitting on the surface, but I have no idea how deep it goes.
The other option is always using polishing compound to do a similar task, but in a different manner. Compounding and buffing actually removes the oxidized portion, exposing non-deteriorated paint below it and buffs a shine onto it's surface in the process. You lose a bit of paint thickness when buffing, but probably end up with more shine (if that's what you're after). The Penetrol finish may look just as good or better on lower-gloss paint, like Kirby's, though it's a cosmetic fix, not a structural one. The oxidized paint is still there (unless it rubbed off on the rag) and it looks better, but it's still a thin layer of dead paint. The Penetrol does not polymerize and re-solidify the damaged surface the way certain polymerizing oils do on wood (like gun stocks with a smooth hard surface). The Penetrol treatment will last as long as it lasts, depending on weather and sun exposure. If it starts getting dull again, give it another coat.
You may even want to adopt some sort of system where you apply Penetrol for a couple of seasons and then use something like 3M 'One Step" cleaner wax every third season to buff off the oxidation (which really isn't doing much good) and start over.
I'm not sure that 303 contains silicone (the msds doesn't say). In their FAQ section they do say to use it instead of products that contain silicone to avoid paint fisheye problems later, which would tend to indicate that maybe it is silicone-free. In any case, it's one of the best wipe-on UV filters that you can buy, so it might be worth a phone call to them to find out if it's suitable for finishes that may eventually get repainted.