1) You have not adequately expressed what the problem is. If the type of plastic and/or rubber you are using is resistant to silicone oil (consult material - chemical compatibility charts) then choose a viscosity of silicone grease adequate for the lubrication needs.
Silicone grease will deter some types of degradation due to sealing off the surface from oxygen to a limited extent, but due to the transparent nature, is not protective against UV damage and in no sense is it meant to repair fissures in plastic or rubber.
You would need to be more specific about what this device is and what components are plastic but generally you can attempt to repair plastic with epoxy but within limitations and rubber is generally not repairable once degraded.
2) Nylon 6 is not UV resistant. Choose a UV resistant material, or a very dark color nylon to reduce the UV penetration, and/or paint it, or install a shield that blocks it from light.
All the info I have given is just generalizations. It is possible the product design is unsuited for the intended environment (aka junk
) and a different product needs to be chosen or for it to be repaired with different components. For example it is not difficult to clamp something with many different readily available materials that are fairly UV resistant, whether you machine them out of plastic or metal, strap with sheet metal, cast with metal or epoxy if you have no more elaborate means to fabricate.
As far as "which product", I do not known what is available in brazil, but one common source for an (approximately) NLGI #1 silicone grease is an automotive dielectric (aka tune up) grease available at well stocked automotive stores.