X-ray Fluorescence (XRF) is a common technique that quantifies individual chemical elements in used oil samples. Samples are typically analyzed by taking an x-ray of a small oil sample (1-2 ml) in a cup. Similar to atomic emission techniques, the large particles associated with abnormal failure modes are not suited to the analysis technique using a cup because the focused XRF beam spot does not statistically represent the large particle distribution in only 1-2ml of oil. These results do correlate well with RDE and ICP; however, the overall elemental signal is much lower. Again, this is expected based on the small XRF beam spot compared to the overall oil volume being examined. Interference from small sub micron carbonaceous soot particles also creates issues for heavily sooted diesel engine oil samples using this technique. These types of samples require some form of baseline calibration to compensate for the soot interference.
You can achieve better sensitivity for large wear particles by focusing the beam onto the particulate itself. This is essentially what occurs when you examine particles from magnetic chip detectors using a piece of sticky tape. The RAF early failure detection centers (EFDCs) in the United Kingdom extensively use this technique.