I've used everything under the sun (do you see what I did there?

) and it all needs re-doing eventually.
My conclusion so far, is if you're going to have to re-do it eventually, just use the cheapest solution out there.
I've tried oils - old engine oil, peanut butter, olive oil... you name it. You just need to spread them (do you see what I did there.. peanut butter...? No?

) so that they are "dry". If you splash it on all over, then it'll be wet.. stain anything that touches it and it stinks.
- Olive/vegetable oil doesn't smell of engine oil and most people have it, rather than peanut butter. Flip-side is that it's food, so it gets mould growing on it if it's warm and damp out there.
- Old engine oil once it's dried and the pong has gone doesn't have any vices... but the plastic will basically have oil on it, so can mark "oily dirt" onto you clothes.
I've tried rubber care products... tyre shine is a good compromise because it's heavier than the "cockpit conditioner" types of sprays... and you give your tyres and door rubbers (to keep frost off) a wipe every so often anyway.. so it's always close to hand.
Of the specialised bumper "dyes".. the Autoglym one is pleasant to use (it's also colourless so not technically a dye) and seems to do what it says on the tin. Smells nice, doesn't mark your white trousers in summer (I know you guys..

) and isn't toooo dear.
One fella got fed up with the whole thing and spray painted his whitened plastic black. You can get specialised bumper paint in an aerosol.. but he later noticed it peeling (a few years I think) so that wasn't perfect either.
Ralf S.