"Can you provide any available standards or suggestions for permissible limits of wear metals in used engine oils?"
As any machine operates, it is bound to produce a set amount of wear debris. How this wear debris is generated will be dependent on many factors, including the lubricant being used, the operating condition of the machine and the cleanliness of the fluid. If the lubricant is not matched correctly to the needs of the machine, it will lead to excessive wear and premature failure. The most common mismatch is choosing the wrong viscosity.
If the engine is operating outside the normal parameters, such as in excessive heat, cold or prolonged intervals without maintenance, this also will cause an increase in wear. Likewise, if the lubricant is contaminated with water, hard particles or any other fluid, it can increase the amount of wear generated as well.
The question isn't as simple as how much metal is allowable. A better way to analyze the results is to trend the amount of metal that is encountered. A single oil sample can provide information on the health of the lubricant, the health of the engine and the contaminants that are present, but routine analysis and developing trends will paint a more accurate picture of what is actually happening inside the machine and the lubricant.
A common way to analyze this is by monitoring the rate of change or the wear rate of the engine in question. Provided the samples are taken at regular intervals, you can use the previous history to see if the wear levels are consistent. If the rate of wear is increasing, this is indicative of a machine that is wearing out.
With engines, you can also determine the location of the wear by the type of metal showing up in the reports. The various engine components are made of different elements, so understanding where copper, aluminum or chromium is coming from can help pinpoint the wear location and the component that is causing the problem.
However, to truly understand the wear mechanism and how far it has progressed, you must run several different oil analysis tests. Normal elemental analysis can detect small wear debris but is blind to large particles. Therefore, it is a good idea to partner this test with ferrous density and other types of analytical ferrography to paint the full picture of the wear occurring inside an engine.