Metamucil is for human, not animal, consumption and the label contains the following warning " taking this product without adequate fluid may cause it to swell and block your throat or esophagus and may cause choking. Do not take Metamucil if you have difficulty swallowing. If you have chest pain, vomiting, or difficulty in swallowing or breathing after taking the product, seek immediate medical attention." For that reason it is not recommended for or given to animals.
Mineral oil is a light oil and because of that it can be easily inhaled into the respiratory tract where it stays. It never becomes assimilated into the body and the body can never get rid of it because of its mineral base. Instead, the immune system tries to isolate it by forming inflammatory granulomas.
"Just having mineral oil around is a liability . If mineral oil is ingested, it will absorb (and essentially rob) vitamins from the intestinal tract – especially oil-soluble vitamins A, D and E. as well as beta-carotene, calcium, phosphorus and potassium and the blood coagulant Vitamin K. This could lead to nutritional deficiencies and other complications.
Ingested mineral oil also can impair lung function, a condition known as lipoid pneumonia (New England Journal of Medicine 1998; 339:1947-1948)."
If your cat is constipated, either a teaspoon or two of olive oil or a tablespoon(level) of canned pumpkin will help to correct this. Olive oil is a heavier oil and both canned pumpkin and olive oil are safe because they do not have a mineral or chemical base.