How best to thin Silicone for Roto Casting
There are various common solvents, like mineral spirits, that will thin silicone.  However, the solvent becomes part of the volume.  Once the silicone cures, the solvent will leach out and evaporate, and the casting will shrink by the volume of solvent added.  This is sometimes done on purpose to make a finished item that's smaller than the mold.  However, the shrinkage can take a long time to complete (days to weeks depending on size), so the results are hard to control if you need a precise finished size.  It can also distort the shape because the evaporation isn't uniform.
I was actually going to try silicone oil but haven't done it yet.  The oil will become part of the volume.  It won't evaporate, but it might leach out over time, producing shrinkage plus an oily mess.  A small amount is supposed to make the silicone softer (which was what I was aiming for), so be aware that it will change the characteristics of the silicone.
Thinning it won't necessarily solve the problem of getting an even coating.  That might be due to waiting too long before getting the mold surface covered, and/or the silicone not sticking to the mold surface (either because it doesn't wet the mold material, or mold release is interfering).  Silicone peels off a lot of materials without mold release, so you might want to experiment with some scrap material to see if you can skip mold release.  If the problem is that it isn't wetting the mold (doesn't stay put after you coat the surface), thinning it with a solvent or oil may make the problem worse.
If it is just a matter of consistency of the liquid silicone, your best bet may be to use silicone that is a better consistency for your purpose.  It comes in different viscosities.