Mineral oil can be used as an intestinal lubricant. It does not absorb water, it resist water. The only way that it can even mix with water is through an emulsifier...e.g. bile acids. Even then, it doesn't soak up water like a sponge. This reaction is only to accomplish absorption. If it is not absorbed, it goes right on through, which is the point. The only way that it can cause dehydration is through excessive diarrhea. This is associated with an overdose, which is entirely possible with such a small turtle. Administer mineral oil with caution, and only under the watchful eye of a Veterinarian! As a Vet. tech, I saw mineral oil used successfully in both mammalian and reptilian patients many times.
Metamucil is merely a bulk-former that holds a lot of water, thereby making the stool more soft. It can also cause dehydration if it is not administered with the proper quantities of water, or if the patient is already dehydrated. Its composition inhibits the re-uptake of water (a necessary component of hydration) while it moves through the small intestine. Bulk formers are beneficial for prevention of an impaction, or treatment of minor constipation.
A bulk former could be extremely deleterious if used in a patient with a bona fide impaction. Mineral oil is a far better choice for impaction.
As always, take recommendations on medical issues from this forum lightly. Always consult a Veterinarian. More work needs to be done before this is even diagnosed as an impaction.
Good luck with your little turtle! I'm sorry that you're starting out on a bad footing with the little guy/gal.