I took apart my starter because it was only turning a little and then would stopped and saw that the brushes were covered in oil and grease left over from when a seal went bad and oil started pouring out of the engine. I fixed the seal, but how would I clean the brushes with out damaging them. I've had suggestions from friends of gasoline or a light dishwashing solution. Are they any good?

I wouldn't try the gas idea.... when it dries it leaves some behind, dish liquid maybe, or carb cleaner maybe. Others will have their take on it.

Brake or Electrical component cleaner.

Try Sea Foam

Seafoams a petroleum product, its light solvents in it may clean a little, but still be oily.

try cleaning them with a zip parts cleaner and then blow drying them with a air compressor.