There are many types of oil that can be used on concrete forms, each with its own advantages and disadvantages. The most common oils used are mineral oil, linseed oil, and tung oil.
Mineral oil is the most inexpensive option and is easy to apply, but it does not penetrate the wood well and will need to be reapplied frequently. Linseed oil penetrates the wood better than mineral oil, but it is more expensive and can be difficult to apply evenly. Tung oil penetrates the deepest of the three oils, but it is also the most expensive and may require special application techniques.
Which type of oil you use on your concrete forms will depend on your budget and how often you plan to use them. If you will be using them frequently, tung oil may be the best option despite the higher cost. If you are working on a tight budget, mineral oil may be the best choice.