Water-based is safest and easiest to clean.
In my experience, failing lube is a sign that the water is being dried out too fast. Rehydrating existing lube should slippery it back up again (rather than just piling on more and more), the question is more one of choosing clean water to do so. Do you wet your dick in a glass? Do you mist it on? Do you dribble it from on high? That part's up to you, just don't use water that's been sitting out uncovered with a chance to have things land in it.
You might also want to see if the drying is happening because of too little humidity in the room. A fan pointed at the two (or more?) of you might help keep you cool, but it'll also dry her out sneaky fast. If you must have a fan, you can either aim the fan up more towards head/torso, or you can keep the sheet/body positioned to block the breeze's direct path.
--Patrick