I'm from Brazil, I work with subsea equipment that requires NAS1638 Class6. We analyse the sampling monitors using optical mycroscope and usually get confused about the size/quantity (every one see things different) and type of dirty (as our equipment have solenoids and don't like metal particles).
I'd like to have more information about this NAS standard. Could anyone help me?

Hello.
You should purchase theARP598 method on how to count the particles (Or ISO4407).

Theese methods specify the preparation, and method on how to identify and count particles.
When you have the correct particle quantity you can use the NAS1638 table to classify the sample cleanliness.

Have you considered using an automatic particle counter with PQ incorporated or perhaps as a seperate unit? While the costs may be higher the benefit of using an apc will give you better particle size distribution accuracy. As you said, the results of optical microscopy can greatly vary depending on the person doing the analysis. And PQ will aid in identifying if ferris debris is present. Decent equipment should also give you excellent repeatability of the results.