You may want to follow suit of the US Military and utilize an in-house oil analysis instrument as a screening tool to determine which samples should be included for lab analysis.
The OilCheck detects and measures the dielectric constant of oil. By comparing the measurements obtained from used and unused oils of the same brand, the TMEH1 OilCheck is able to determine the degree of change in the condition of the oil.
Dielectric change is directly related to the degradation and the contamination level of the oil and will allow the user to achieve optimized intervals between oil changes and detect increased mechanical wear and loss of the oils lubricating properties.
http://www.maintenanceproductsdirect.com/Products_Instruments/SKF-TMEH_1.htm