white flakes -Like dandruff, when rubbing coat

Question
Clara Belle, german sheperd-rotweiller mix,exactly
50-50. Her fur is short like a rotweiller. The last 2 years I noticed white flakes when I rubbed her (she is 5 and didn't have this problem before)
Also it seems like she shed more hair, she has a nice coat of hair though. Vet reccomended adding a tsp of oil to her food, but haven't done that yet, thinking she will have to go all the time(potty) I feed her Purina 1.  

Answer
1 tsp of oil in her food per day is NOT going to make her potty more.It would take quite a bit more than that.
I would suggest cod liver oil. It has more vitamins in it than cooking oil. The oil from a can of tuna would be good too.
Another thing that could be a factor, how often do you bathe her.
If you bathe her more than once a month, you could be contributing to dry skin.
Dogs don't generally need a bath, unless they roll in something. A daily brushing is usually enough to keep them clean.Your vet and/or the pet stores carry some good skin conditioners. some are spray on, for dogs that are brushed, rather than bathed. More bathing requires more conditioning.
Does she scratch a lot? Scratching may indicate allergies.
What kind of dog food do you use.
If you are using one of the cheaper brands, there may not be enough nutritional value in it.I would recommend switching to a better brand, maybe Science diet or Iams.
If you switch, do so gradually. Start with 1 part new food, and 1 part old food. Feed for one week, and then 2 parts new food to 1 part old food, for one week. Third week, feed all new food.switching too fast can cause Diahrea or constipation.
When you start adding the tsp of oil, she may go potty a tad more for a few days, or it may be a litle looser., but when she adjusts to the difference in her food, that will take care of itself. If she has diarhea, then of course, she should be seen again, or at least call her vet for advice.
Her age is a small factor. At 5, she is not that old, but rotties have a shorter life span, so that is in the senior age for a rottie. she may have more rotty traits in that area. she should be on a senior formula of food. Science diet, I ams, all those and the better brands,like Purina, all have senoir formulas. They are geared to an older dog's needs for nutrition. Older dogs need different amounts of some vitamins and minerals.They are less active than a younger dog. The right formula for your dog's age will help them live a little longer. The senior formulas have less fat calories in them.
I am sure if it were Mange, or some other skin disease, your vet would have diagnosed it.
Follow your vet's recommendations. That's what you pay him/her for.
Good luck to you and your girl. Let me know how she makes out.