With the change to pasture is it further to get to water than it was in her paddock? Has the type of water container changed? If so, it may be that the water also tastes different. All of these things can cause a horse to stop drinking as much. They may not like the shape/color of the container (especially older horses who's eyesight may make certain shapes/colors/depths hard to differentiate). They may not like the flavor of the water if it's not cleaned and refilled as often, has grass bits in it, etc. They may also not want to walk as far to get their water if they have arthritic issues.
So start by looking at those changes and maybe make water available in more areas and in containers like the ones used in the paddocks.
As someone already mentioned, vegetable oil will not lubricate the GI tract. And lubrication may help with the already present hard feces. However mineral oil would need to be fed in quite large quantities to do much. You are better off getting more water in her.
You may also want to reconsider her diet. Depending on how much rice bran you are feeding, you may be really throwing off the calcium to phosphorus ratio in the diet. That can affect the overall health of your horse. Rather than making a mix-n-match diet, you should consider going straight to the label recommended amount of a complete senior feed. A balanced diet is extremely important to help maintain optimum function of all of the body systems.