substituting one oil for another. In general, you can substitute any oil for another oil in most recipes. You will change the skin feel, viscosity, colour, and other features, but you won't change the chemistry of the product. (Meaning that your lotion will still emulsify well...) For instance, if you were previously using meadowfoam seed oil - a very light, less greasy feeling oil - for olive oil - a heavier, medium greasinesss oil - your lotion may feel slightly heavier and greasier. If you were to substitute the other way, you'd have a less greasy, lighter feeling oil.
There is an exception to what I've just said. Castor oil and beeswax have a neat way of interacting that makes the product more plastic, which is why this combination is used a lot in lipsticks. If you see castor and beeswax being used together, it's probably a wise idea not to alter the recipe unless you know they aren't working together in this way. (See this recipe at Voyageur for making non-petroleum jelly with castor oil and beeswax. You cannot substitute the castor oil for another oil in this recipe!)
And yes, you can freeze your oils!
I encourage you to get to know your oils by playing with them. When I do my lotion making, facial products, and anhydrous classes at Voyageur Soap & Candle, we set up an oil bar where people can play with the oils to get to know the skin feel before we make our products. I think this helps to make more educated choices about the oils you're planning to use, and I think it saves you money because you don't need to buy tons of different oils when four or five will do. (The picture above is of our oil bar!)
On a secondary note, when substituting one oil for another, there are many ways to determine how to do it. You could compare fatty acids, greasiness level, viscosity, cost - there is no right way to compare oils. It's about what you want and the application in which you'll be using it. I encourage you to visit the emollients section of the blog to see detailed posts about loads of different oils and to see the comparison charts I've written.
Getting to know your oils - scroll down to see the start of the series