I will second the use of "303 Protectant". It is supposed to be a good preservative for everything. I have seen cautions against the use of some silicone sprays, saying that there may be other substances in the propellant that will harm the surface. If the propellant is inert, silicone will be OK but perhaps the "303 Protectant" will form a better barrier that will repel other substances and prevent the loss of vital juices. It's not cheap.
I have also successfully used the Mequires white/watery substance for inside the car. Until it cures, it is very susceptible to water, causing streaking. May not be the best to use under the car!
OliverB