When using RTV, so long as it's the common Permatex type, you can add fluids imediately unless the instructions on the product tell otherwise.
The portion of the sealer that is actually doing the sealing is in between the casting, and most likely will never come in contact with the oil, especially since both halves are machined surfaces.
It takes very little RTV to seal such surfaces, most guys over do it, it only takes enough to fill and voids in between the two surfaces. I've taken things apart where someone has globbed on so much that it never really cures or the edge that's formed come free and gets into the works. This is a real problem on anything with an oil pump or screen. It's not as crucial on something like a rear differential cover.
Myself, I prefer to use an anaerobic sealer like Loctite 518 on any metal to metal situations. There are several brands of this type of sealer. This is closer to what the factory used when new.
Sealers like Loctite 518 are also sometimes referred to as hydraulic sealers, since they work by forming a hydraulic, air tight seal between two machined surfaces. This is also a longer lasting more durable sealer.
Here's a link that better describes when to use each type of sealer:
The only time that there may be a problem with imediately adding oil is on a diesel engine, where as the type of silicone and type of oil and it's additives are also a concern. (Certain types of RTV silicone can effect the antifoaming ability of diesel engine oils).