And just as a doctor can order blood work to pinpoint health issues, engine oil can be checked for unusual concentrations of metal particles, dirt, seawater, antifreeze or other contaminants that indicate specific problems. "Testing gives you the condition of the oil and, indirectly, the condition of the engine," said Charles Boswell, president of R&G Laboratories in Tampa, Florida. A high concentration of stainless steel, for example, indicates a shaft is wearing. Copper comes from excessive brass- or bronze-bearing wear. Chrome warns of piston ring problems, while tin raises concerns with cylinder walls. Contaminants from outside the engine are also important indicators. High sodium, for example, shows that seawater is finding its way into an engine, while potassium indicates that antifreeze is leaking internally.