On some kinds of plastics silicone oil does support a nice look without doing harm to the structure. It is known and used for the effect to avoid anything else to adhere to the surface covered with silicone oil. However, some plastics might be damaged severely. In any case silicone oil turns out to be a real mess as soon as one actually wants something to stick again to a part formerly treated with silicone oil.

We know how different Macintosh case plastics age. Often enough Apple used different batches of plastics for different parts of the very same machine. Some become brittle, some become brown, some stay as they are. PVC or Vinyl is not common in Macintosh cases. We find ABS, PC PMMA and other blends with a variety of fillers, colourants and flame retardants. None of the ingredients will interact chemically with the silicone oil, but the silicone oil spreads completely on the whole surface and into each tiny little crack. It changes the surface energy strongly and will stay permanently. Even sanding the surface will not completely remove the silicone oil as long as some of the original part is left.

It is strongly recommended to do some experiments with that stuff before an attempt to contaminate valuable things with silicone oil. If it works for you, it is fine. Final it will be, also.