Shock oil or silicone is designed to be used in shocks (no kidding, right?). The real speciality of the oil comes in the specifications of weight or viscosity. Knowing this information allows a person to fine tune their shock set-up.
Could you use any old oil? I suppose an arguement could easily be made in the positive, but I would ask why not just spend the five bucks or whatever it costs to get something you are certain will be work, has been tested and is proven to work?
So, can you use motor oil? Can you use differential oils? Can you use household oil? Can you use ...? I guess the answer is yes. The only thing I would caution is that some oils have additives that are not rubber or silicone seal friendly. These oils might eat the o-rings and seals that keep the oil within the cylinder of the shock. That would kind of defeat the purpose.
I guess I should have been more forthcoming with my post.
I was combining it with the post above mine that recommended you use hobby shock oil.
If you would like to let us know the RC you have and where you plan on running it we can help you select the proper weight range of silicon shock oil that will be best for you.