But some of the smaller studies conducted over the past two decades show that no sperm is "progressing" and that some doctors have begun to say that it will not lead to pregnancy. Pediatric rheumatologists are doctors who specialize in providing comprehensive care for children (and their families) who have rheumatic diseases, especially arthritis. L-Arginine is a non necessary amino acid and is essential for regular performing of the pituitary gland. Used motor oil which has impurities and dirt is fed right into a centrifuge. The centrifuge carries a rotor that spins an incredibly high RPM (revolutions each minute) and it pushes the impurities to the wall in the container that houses the rotor. This works because the mass from the impurities is more than the mass with the oil particles. Appropriate extraction methods are used to ensure that impurities along with the clean motor oil are properly separated, and the oil is able to be re-used. These filtering systems can also be suitable for use with small-scale biofuel setups to get rid of any impurities.