Greg,
ISO 4406 provides guidance in charcaterizing the cleanliness of lube oils. It is based upon counting the number of particles in given size ranges (>4 um, >6 um, >14 um) in a one ml sample. This methodology has been adapted for use by some equipment manufacturers.
NAS 1648(National Aerospace Standard)provides similar guidance but uses a single characterization number to describe the particle size count and distribution in a 100 ml sample.
Regards,
Gunnar