I tried silicome spray before. After it goes on the part looks great and better than new. But come back the next day and it looks like you never did anything. Like you wasted your time. Silicome sprays can also degrade plastic.
Rubber conditioner, tire spray, armor all and even dielectric/silicone grease would be what I would do. You should get good cloths that are soft enough to not tear the bushes, but strong enough to not tear in half.
It can be quite a process and if these parts have been untouched and the car is old- it can take some going over.
First they need to be clean. You could clean them all a few times until they look good and the towel stays clean.
Then, second, they will need the conditioner applied. It usually wont absorb it all at once- so (as you said) do it once a month until they look good and no longer dried out......or you are tired.
Then you should probaly use water with a few drops of car soap the next few times to get the excess rubber conditioner and the dust and dirt it has attracted off of there.
Then, after that.....they should be clean and conditioned, BUT DRY so they wont hold onto dirt so bad and you wont have to keep doing this unneccisarily.
So, clean, apply conditioner, then clean off the accumulated dirt and excess conditioner. So they will be treated, but dry and not attract road debris in the future and require ongoing attention.
