The last oil sample was taken in duplicate. Normally I do this in case it gets lost in the mail to the USA.
This time, one was sent to Avlab (my usual firm) and one was sent to Blackstone.
Avlab:
Blackstone
Both were taken within seconds of each other, from a warm engine (just after a flight), and having waited for 1 minute from opening the oil drain.
I have asked both firms to comment.
Update:
Both companies commented, each saying they don't know anything about the calibration procedures the other one uses.
Surely the whole point is to spot trends, and in that regard absolute values aren't critically important
Yes, although excessive values (the exact threshold for concern is debatable) are a cause for concern.
Just like PSA, really 
